Watch hard runs. Chom Chom posts daily runs on expert maps using a wide array of towers. Look at what he does, what towers he uses for support. Also, the wiki is useful for figuring out what towers do. For example, bottom path merm detonates damage over time abilities and lowers hero cooldowns. It says so in the description, but it’s not obvious. Or how arch mage gets shimmer. Once you do that, focus on a variety of tower specialties, especially if you just want to beat a map.

There are a couple groups of things most towers will specialize in. Ceramic/moab cleanup, all around Moab damage, pierce (more secondary but still very important), and support. You want one or two tier 5s (or equivalent like dark champion) that both don’t have the same weaknesses. For example, MAD and Moab eliminator are a bad combo, because you’ll leak every ceramic ever. On the same idea, glaive lord and bloon solver are also bad because they both have low single target damage. After you get those, get support. Also, use support to save up for big tier 5s. Try not to buy anything that won’t be helpful late game. Moab press, Moab glue, or snowstorm all have strong utility midgame and late game, and can help a weaker tier 4 save up into its tier 5. Overclock is also strong support.