Everything takes practice. Did you win the Tour de France the first time you rode a bicycle? Nope. Did you win the Indy 500 the first time you got behind the wheel of a car? Probably not. The important thing is to figure out where you're having problems and discover how to fix them. Common woodworking problems include:
1) Poor measurement. Solution? Buy some good measuring tools: a FastCap FlatBack tape measure, combination square (e.g. Starrett 12"), and maybe a dial caliper. Better yet, create a story pole so that you don't have to measure anything!
2) Inability to cut straight. Solution? Build a sawboard or buy an edge guide. You'll also want some clamps to keep it in place while making cuts.
3) Problems with math. Solution? Buy a calculator that can handle fractions (e.g. Construction Master) if you're having problems with math (12 3/32 + 14 5/8 =

??).
4) Inability to create or read plans? Buy a step by step book (e.g. saint's) that will lead you through things from start to finish. Follow the book
exactly. Once you have a successful project or two under your belt
then you can start to branch out.
And of course you need to learn how to recover from your mistakes. If you learn how to sharpen it, a low-angle block plane is great for helping to recover from mistakes. It'll remove wood in small amounts until you have two pieces that are exactly the same length or width. In most cases, it doesn't matter if your pieces are some exact length, but rather that they're all the same length...