Experience. Start with the simplest of things: replacing a standard (non-GFCI) receptacle. Buy a book that tells you how, or learn from someone who knows. (B

Experience. Start with the simplest of things: replacing a standard (non-GFCI) receptacle. Buy a book that tells you how, or learn from someone who knows. (Both is best.) Don't guess at anything. If the book doesn't say exactly what to do in the situation you're in, hire an electrician. (This sounds overly cautious but really – if you're doing something even slightly "novel" then there's either going to be code issues you aren't qualified to judge, or actual electrical issues you aren't familiar with and don't even know to think about, like grounding issues.) Thankfully, most situations a homeowner will encounter aren't left up to interpretation by code, and are therefore covered in a good book. Move on to similar things (say, replacing a simple switch or GFCI receptacle) when you feel you have enough experience.