I actually went to my one and only Comic-Con in 2003 after I graduated from high school. I have an aunt, uncle, and cousin who live in Coronado, which is a 15 minute drive from the convention. I got very lucky that they were nice enough to put me up for the weekend, feed me, drive me there to drop me and my cousin(their son) off and pick us up, which saved me a SHITLOAD of money on hotel, food, and parking(plus I don't know the law, but I don't think a 17 year old, which I was at the time, would be allowed to get a hotel. Think you have to be legal to rent a room). If I had to say some suggestions, I would say:
1. Do not leave your home/hotel without a fully charged cell phone, especially if you're going with someone else. You'd be amazed how easy it is to get separated from people, and a fully-charged phone is a MUST when going onto the floor. Without that phone, I'd probably still be wandering around looking for him. It can be a bit scary out there on the floor, so having the phone is vital.
2. Set a limit on how much $$$ you are going to spend prior to entering the building. This will not only save you from spending too much, but will aid you in avoiding buying random shit and saving your dough for something you really want.
3. If bringing a digital camera, bring backup batteries.
4. Eat before you get there. The food really is outrageously priced.
5. Have fun! It can be very stressful with the crowds and the lines, but overall, it's supposed to be fun. I had a ball in spite of how stank some of those nerds were. Fuck, they STANK! But it was a good time. I made friends at Con that I still keep in contact with. Most everyone there is there to have a good time and hang with likeminded folks. There wil be some irritable dick there, but avoid them and you'll have fun. And I hope you do have fun. Good luck!