When you can buy an upgrade, do that. It might not seem impressive at first, but it will get there as you continue to improve it. However, you should start with two upgrades in particular: the flat sum tap bonus first, and the percentage increase bonus after that. The reason is that those will give you more bang for your buck, so to speak, so that every action you take thereafter will yield greater rewards than it would have. After that, just go for the updates and upgrades as they come your way, without worrying overly much about the order.