It can be difficult to get a straight hole when using a hand drill. Some drills come with a level mounted on the back, but Popular Mechanics shares a few other tips for drilling straight.
If you don't have a drill press handy and need straight holes for, say, creating holes for dowels, you can use an old CD and drill through the middle. Even better is a quickly assembled jig:
Nail together a couple of 1 x 2 scrap pieces to form a right angle. It's important to make everything true and square or you're wasting your time. Position the drill bit tight into the corner of the jig and bore your hole.
Heck, if you don't have scrap wood, you could even use Lego. See the complete list of tips at the link below.
3 Tips for Drilling Straight Holes [Popular Mechanics]