You pick your own number. Check out the results page on this site and you will see what the numbers are that others have already chosen. Two digit numbers are easier to read so I would stick with them.
Yes, you may enter your first race on the 13th at IMI.
The only qualifications is the safety of your kart. If possible, I would do the pre-tech on saturday so as to give you time to correct any problems you may have.
Weights are posted on this site. Your kart should weigh some where around 180 to 200#. Add to that your weight with safety gear and that will give you a rough idea where you are at right now. The scales will be up at IMI in the morning and you will be able to weigh then. Be ready to add weight as required. You can have the seat drilled before you get there and have your wieght and bolts ready to go so it will be a simple operation to add sunday morning. If you ask nicely, I’m sure Bandi or IMI will power up the scales to give you a complimentary weight check. However, it is always wise to double check the scales and you to see if the scales are still where you think they should be.