Tying your tubes is permanent, and not meant to be reversible. You would be extremely hard-pressed to find a doctor that would allow an 18 year old to have her tubes tied.
If you don't have a regular GP or gynecologist, check out your local Planned Parenthood. Even just the website contains a wealth of information.
To get ANY form of hormonal birth control, you will need a pelvic exam and a pap smear. If you are not good about taking pills, Nuvaring, the contraceptive patch (only works if you're below 170 lbs though), the Depo shot, and Norplant are all great hormonal alternatives.
For non-hormonal, there is always the old fallback of condoms, which most Planned Parenthood places will give you a giant box of if you ask for it. IUDs (IntraUterine Devices) last a long time, and come in hormonal and non-hormonal. They used to be only for women who had had children already, but they now make ones that can be used by women who have not gone through childbirth. They're expensive, but are basically "set and forget" (Norplant is like this too), and last for years.
What you choose is going to depend on your finances, your body's reaction to hormonal birth control, and if you are able to keep up the maintenance required by some of them.
Personally I am looking at getting back on the pill. It's a bit of a PITA, but for me it also regulates my periods and hormone levels (I used to have REALLY horrible PMS symptoms to the point where I would have extremely severe mood swings).