Take voice lessons. Having a vocal coach can literally take you from a caterwauling cat to a controlled belter, if you give it enough time. If you practice consistently (and your coach is legitimate), you will notice marked vocal improvement within a few weeks. Not only is this important for the strength and overall quality of your voice, but you can also damage your voice if you aren't singing properly; having a vocal coach, in theory, prevents that.
If you can't afford voice lessons, try a learn at home voice training program like Singing Success or Sing for the Stars as they can teach the basic foundations of voice technique.
Before you write singing lessons off due to financial woes, know that most coaches try to drill in a specific method and will establish that within the first few sessions. Once you know how to get the results you're looking for, you can, in theory, practice on your own.