Look the part. Or, as the saying goes, "fake it 'til you make it." If you know that you look like a confident, capable person, eventually you'll start to feel it, too. You should dress how you feel best -- not what you think confident is. Try these tricks:
Devote a little time each day to personal hygiene and making sure you're presenting yourself well. Shower daily, brush and floss your teeth, and groom your skin and hair.
Dress for confidence. You don't have to buy a whole new wardrobe to feel better in your clothes. As long as you're clean, comfortable, and feel good, you're set up for confidence. After all, you wouldn't wear a three-piece suit on a pizza delivery. If you think you look good, odds are you probably do. Be careful that you don't overdo it (e.g. putting on tons of make up or wearing too revealing clothes). Dressing appropriately for situations gives you one less thing to worry about.