Game design can be hard at the micro level.
How much xp should it take to go from level 1 to 2? How about from level 9 to 10? Should this weapon do 25 damage or 30? How far should this unit be able to move? These questions are even harder if this is your first time making a game.
These questions can often be answered thru a lot of playtesting, but there is a way to save time.
Borrow from other games that already work.
I am working on a collection of turn based games.
Once common concept across these games (since they are ultimately meant to be joined together) is the idea of characters that can gain experience and level up. Instead of trying to design this very common system from scratch, I am borrowing ideas from existing games that do things the way I already like. They already did a majority of the heavy lifting.
Now I just tweak values to my liking.
Keep getting wiser, stronger, and better.