You do not have to choose a persona right away. Attend a few events first, try out different garb and talk to all kinds of people. You may find something more interesting than you originally planned. Once you pick a name and people start calling you by it, it is challenging to change it as that is who we all met you as. One way to avoid this is to pick a first name that was common to several areas you are interested in, and decide on a last name at a later date. You cannot use the name of a real historical person or a fictional character, or a name that implies a relationship to either. You cannot use a title which has not been awarded to you within the SCA. A good web site to check out for ideas for and the rules of SCA name construction is the Medieval Names Archive.