Right from the beginning of the game, you'll earn a random hero, but you can reroll for a different, random hero. You've probably seen references to rerolling but are unsure what it means. Make sure that you log into the game as a guest, NOT on Game Center; if you have already logged in via Game Center, log out and log into a different account. Then delete the app, reinstall it, then roll again. Keep rerolling until you get a better hero; three-stars and four-stars are possible. On Android, change your settings for games so that they do not auto-log into the game when you play them before firing up Injustice 2 for the first time.
Gems are the premium currency of the game and are available for purchase, but you can also get plenty of them for free. Use your gems to purchase hero chests, especially early on in the game. Hero chests give you a random hero, with a fairly decent chance at a three-star hero. If you get a duplicate hero, you'll get hero shards instead. Another way to go with the gems is to grind in the store for specific hero shards. Can get expensive, but do this and you can be sure of what you're trying to unlock.
Hero shards are used for upgrading a hero's star ranking. Go to the hero's profile and you'll see how many shards you need for the next upgrade underneath the star ranking. Once you max out the shard count, you will be able to upgrade your hero's star level. Consequently, once you have enough shards to unlock a rare locked hero, you'll be able to unlock them.
Hit the arena and battle it out in exchange for arena points, which earn you new rewards, including locked heroes. Be sure to use only one hero at a time and to round to the nearest 2,500 or as little above as possible (e.g. as close to 2,501 or 2,502 as possible without going below). Each hero comes with 5 pips per day, which is arena energy, so you can fight 5 battles with one hero at a time for 15 battles with three heroes, or you can fight 5 battles with three heroes at a time and blow out all 15 of their combined pips in one shot.
If you want to spend your gems for the future, go to the objectives and upgrade them like crazy to increase the chances of earning objective gems. Upgrade them high and you'll find yourself earning 30 gems from random short operations and 100 gems from random long operations. You can also earn hero shards from the operations themselves, as well.
How to load up on free gems
Early on in the game, all of your free gems will come from completing objectives, which usually are unlocked by doing something repetitively, such as winning five arena fights. Run down the list of objectives and complete the ones that are the easiest to complete. Fighting the story mode battles, in particular, will load you up early on, as each one completes a 50-gem achievement upon finishing the battle.
Once you do that and you have plenty of gems to play with, go to the operations area and spend gems in order to upgrade your operations. Load up until you have a significant percentage boost added to your gem earnings. The higher that you upgrade your operations, the better your chances of earning gems and the more that you'll earn.
Fighting in the arena is a great way to earn more gems, along with other bonuses. The more you fight and the higher than you rank up in the arena, the better the rewards that you earn. Rewards get doled out as each tournament session ends, which is weekly. Jump into the Elite ranks and fight to keep your place and you will earn free gems.
Even if you don't plan to play much for the day, log in once every day anyways to receive your daily reward. Many of the rewards are gems, and at the end of the calendar, you'll get a gem jackpot. Plus, complete your daily quests, as well. The standard quests don't always give you gems, but the daily ones will most frequently give you gems.
Plan for the future to get more overall gems. If you are going to make an in-app purchase, get a card or an elite card. These will not pay out all of their gems at the same time, but they will pay them out gradually and they will pay FAR more than will spending the equivalent amount of gems for the lump sum total. It's like a gem trust fund.
Various Gameplay Tips
Each hero tends to have their own strongest combos, but as far as pulling off as many hits in a row as you can, your best bet is usually either dash-tap-tap-tap-dash, or dash-tap-tap-tap-jump. These act as an unbreakable combo as long as the first hit isn't blocked, and they will rapidly fill up your special move bar, in addition to doing high levels of damage. Add a special move onto the tail end of the combo to not only do massive damage, but to ensure that it lands.
Start practicing blocking and counterpunching early on so that you're ready when the opponents become tougher to beat. Anticipate the coming attack and hold block, wait until the opponent's hit or combination is blocked, then immediately hit them with a combo attack before they can attack again.
When you unlock the arena (which happens at level 3), a good tip is to place only one character into the arena instead of all three. The tier of opponent you face is based on your total team threat, so your team threat will be very low compared to who you're playing against. This maximizes the efficiency of pips usage and allows you more practice with a specific character.
You can send unused heroes out on operations in order to earn rewards, such as xp and gems. Level up your operations with gems in order to earn more gems whenever you complete them. If you're stuck somewhere in a campaign, send your primary heroes on an operation instead, then let the game sit for awhile while they complete it, and they will gain big boosts when they come back, strengthening them and breaking through that difficulty barrier.
Keep multiple classes of characters in your party for the campaign mode (and the arena if you choose to use all three slots) so that you always have someone who is a type match for whoever you're fighting against at the time. Don't make a party out of homogenous heroes unless your heroes are so rare and powerful that the type match doesn't even matter at the time.
Gear is one of the more complex bits of the game so far. Gear can be bought in the arena store and earned in other ways. Each character has different gear sets, and stat bonuses are earned if you equip them with either two, three, or five different items from one single set of gear. You can shatter unused gear into fragments, then use it to level up and upgrade the gear that you want to use.
Aside from the operations, one of the best ways to earn free gems is to complete the objectives that the game gives you. This doubles as a great way to level up your account as well. Take those free gems and open hero chests until you get a three-star (four stars are very rare but you can try for one of those too). Once you have a three-star or two, your best bet is to spend them on upgrading your operations.
Once you complete all of the standard battles in a set in the campaign mode, you can unlock the heroic battles. Be careful before you try to complete these, because they can be extremely tough, so you might need to upgrade your team and its threat level first. You can try heroic battles three times a day, and because they are tougher to beat, they will also give you much better rewards than the corresponding standard battles will.
Rank is gained or lost in the arena depending on whether you win or lose, and what your team threat level is. You'll need to experiment to figure out the best team combinations for maximum rank gain, but if you are using all three characters (or two very powerful characters), round your threat level up to the closest 2,500 points to earn the most rank from victories. Rank points increase by 250 for every 2,500 threat level passed (2,501, 5,001, etc).
How to get star heroes...