Are you looking to just attend, set up a table, or sit in on one of the panel discussions?
Attending is just a matter of paying the fee.
Getting a table is again, just a matter of paying a fee.
Getting invited onto a panel to discuss topics with the fans, or maybe even do a reading, is generally by invite only.
Very few first book authors get those, but it is not unheard of if the book does well.
For most cons, members who sit in on a panel get comped with free room and maybe meals, a table, and maybe some free tickets to give to friends and fans.
Contact the various cons you would want to attend and see what they offer. Couldn't hurt...