Unless you are going to kill everyone it is more beneficial to not spill any blood as being a pacifist is the only way you will get the 'true' ending. Also, by not using your weapon you will find the vast majority of your encounters with enemies a lot easier.Taking this approach also allows you to explore extra environments and unravel extra story details you'd otherwise miss out on.
Don't Spoil It!
Resist the temptation of researching the game and flooding your brain with the various fan theories on social media until after you have played it. Doing this will enable you to experience first hand the various surpises that are thrown at you as the game hooks you in.
Explore
There is a lot hidden in this game so make sure you explore everywhere for secret paths. Take your time and talk to every vendor at least twice and revisit areas you have already explored after key in-game events in order to see some of the game's more amusing escapades.
Medical Supplies
As there is not a lot of loot to be found in this RPG you will have to purchase most of your supplies. Make sure you make healing items a priority and keep a full stocked inventory on you as much as possible. Until you reach the half way stage of the game you will be unable to sell anything so you are better of throwing away old weapons and armour in favor of making room for medical supplies.
Take your Time
Although it is easily possible to complete the game within a couple of hours you need to take your time and talk to everyone you encoumter and study everything you see if you want to get a full sense of the world you are in.
Undyne Boss Fight
Remember when you encounter this boss that there's no Act you can do to help with this fight, on every turn you need to just attempt to Flee. Even though Undyne will catch up with you a couple of times you need to keep on fleeing and eventually you'll lead her to Hotlands where she'll collapse from exhaustion.
Be a Pacifist...