There are no dumb questions here!
First off, depending on the con and their policies I would highly recommend pre-registering as much ahead of time as you can...you'll generally save a few bucks that way. Regardless, I recommend pre-registering in order to cut down on the amount of time that you wait in line.
If the con is in your city, I would say skip the hotel room and save the money. However, if you do have the money and/or friends who want to split the costs, I would definitely recommend getting a room. as it really makes the con feel like a mini vacation and allows you to fully immerse yourself in the con from beginning to end
As far as what to do? Well, there will be lots of cosplayers so be sure to bring a camera. Always remember to ASK FIRST as common courtesy. 9 times out of 10 cosplayers will be more than happy to pose for pictures but if they turn you down for whatever reason, respect it and move on...do not demand. And do NOT barge in a private photoshoots.
Also, there are all sorts of panels and screenings going on and perhaps even concerts as well...always check the schedule to see what is happening!
Lastly, I'd seriously recommend cosplaying if you are ever able to do so...it is so much fun and it really enhances the con going experience