welcome - take a moment on the introduce yourself forum to say hi.
onto the topic at hand - any caliber over a .380 would be recomended.  there are a host of other questions to consider:  
how often do you plan to carry?
what is the largest caliber you can reliable hit with in a conseal size gun?
how much you want to spend?
how much background you have with weapons?
i personally own 3 different carry guns.  which one i carry depends on what i am wearing, the length of time i intend to carry and where i'm going.  that's how i roll, othere people are different.  
 start with asking yourself some of the questions above.  once you have a purpose/picture of what you want to do, i recomend visiting either of the Bill's guns locations.  talk with the dudes at the range.  rent a few different guns.  
hold them, see how they fit your hand.  
shoot them, see what they do when you pull the trigger.
this will give you a MUCH better idea of what you want to spend X amount of dollars on and more importantly what gun you want to rely on should your LIFE depend on it! 
this isn't a cut and dry question and it truly does take some time to find something that works for you for what you want it to do.  be patient, learn as mucha s you can and PULL SOME TRIGGERS FIRST!  WOO WOO!!!  

also +1 on recomending Thunderjohn to deal with.  A class act all the way.