When I was starting out I bought some of the basic colors (black, white, pumpkin brown, pumpkin green, chartreuse yellow, and plum purple). I’ve tried a few different brands over the years and I tend to favor the x2 do-it followed by m-f in the non-bleed versions.
Start small and try mixing/blending your basic colors to see what you can come up with and how they play in the plastic. Then expand to other colors after you get comfortable.