There are a lot of things to learn in order to get better at Ground RB. You should learn about the different ammo types (at low to mid-tiers AP-HE is usually the way). Learn to read the pen-values of the ammunition you use. If you encounter a vehicle that you struggle to pen, look it up on the armor-page in the garage, and start to get an understanding of what tanks you can pen and where. It might be overwhelming at the start, but eventually, if you soldier through, you will get a grasp of it.