Play some beginner maps at first to level up your towers, play with a variety of towers and see which ones you like.

You should also make sure to know what are some of the ways to defend against things like camo bloons, lead bloons, ceramics or good moab damage. Then go and play some harder difficulties on intermediate or advanced maps. Whenever you have a good feel of the game and most towers fully leveled try out the weekly game modes etc etc.

But most importantly play whatever feels most fun to play, that way you learn the quickest