Have a fairly good singing voice. Technology can do wonders these days, so you don't have to be opera-worthy to make it on the charts. You should be able to do the basics, though, so if you can't carry a tune, enlist the help of a singing coach to get you to a higher level. Britney Spears has studied since she was about five-years old.
Show a little skin. If you want to sing like Britney Spears, you have to ooze sexuality with every breath. Show off as much skin as you can, and sing about things that go beyond just a little naughty.
Get moving. No one would buy concerts tickets if it meant watching Britney sit in a recliner chair while she belted her tunes. Put on a wireless microphone and work it. Dance and jump around while you sing; it keeps your adrenaline pumping and your fans excited.
Play the part. Being able to sing is only part of the Britney Spears equation. The reason she got to the top is her showmanship. Think fireworks, a 100 foot-tall Buddha statue and hundreds of choreographed backup dancers. The bigger the performance, the faster the tickets sell.