Tying your tubes is no easy thing, and it really isn't reversible. It's more of a permanent thing, and if you ever DID want to "undo" it, Insurance won't pay. So plan to have tens of thousands of dollars available. Also, It's MAJOR surgery. And no ethical surgeon is ever going to perform one on an 18 year old "just because"...
You really need to go see an OB-GYN and talk to them personally about what types of BC will be best FOR YOU.
With the pill, you really need to be responsible and take it every day, at the same time, every day. Certain antibiotics can cause the pill not to work, so if your ever sick, you will probably have to abstain or use a condom if you do have sex. (Depending on the antibiotic prescribed)
Don't do an IUD. At your age, never having sex, and never having children, there are too many risks involved. It's possible to puncture the uterus, and then you'll never be able to have kids.
Depo shots are okay. They last for 3 months, so you only have to go get them 4 times a year. I used them for awhile, but I seriously gained ALOT of weight on them. It is progesterone only, and can cause a hormone imbalance causing weight gain. It doesn't happen to everyone, and they "say" you only gain around 10 pounds, but seriously...No. I gained alot more then just 10.
I currently use IMPLANON. It's a small plastic rod that is inserted into the underside of your upper arm. It lasts for 3 years. Has very few side effects. And I never have to worry about puncturing my uterus, or missing a pill, or having any medications interfere with it's effectiveness. I LOVE this and recommend it to ALOT of people. Official website for IMPLANON® (etonogestrel implant) 68 mg
There are other options. Nuva-Ring. Diaphragms. Condoms.
But you really need to find something that works with your schedule, that your comfortable with, and won't be detrimental to your future plans. (kids)