If you know where the mistake is, one technique you can use is to purposefully drop a stitch above it, let it 'ladder' down to the mistake, and then pick up the stitches that are supposed to be there with a crochet hook. The other way is to undo your work - either by undoing one stitch at a time (which is good if the mistake was in the same row) or taking the needles out and just pulling the wool out quickly (i call this 'shredding') until you get to the mistake and then picking up the stiches again (if its a long way down, this is rather depressing)
The best advice I can give a beginner is to learn how to 'read' you work. Make common mistakes on purpose and then see how they look in your work. Some of those common "mistakes" end up being advanced stitch techniques.
And lastly: patience. We all had to start from somewhere, and i think we all went through the 'growing work' stage. And please be reassured, I've been knitting for nearly 35 years, and I still pick up the odd stray stitch from time to time - it just takes a moment of inattention and there it is. Its fairly easy to fix if you are doing garter or stocking stitch in plain 8-ply, not so easy when its an intricate lace pattern with the sort of yarn that is so fuzzy that its designed to felt to itself
Just keep practicing, it will come.