Hello. I love when people take the initiative to get help when they feel they need it. It's great!
Because you're 18 you can make an appointment with your local doctor/GP, and basically tell him the symptoms you're having. Because you've been feeling severely down for a long time, this counts as a symptom of depression. Also things such as losing interest in once favorite activities, weight fluctuation or insomnia/hypersomnia.
Once at least 5 symptoms have been established normally the doctor will ask you if you wish to try anti-depressants, and usually he will prescribe you for between 1-3 months worth of pills.
If he doesn't suggest this then you can ask him if anti-depressants would be suitable to your current situation.
Because you are at university I assume they would have a counselor there? Or some sort of program that helps people deal with the stresses of university and such?
If not you can always ask your doctor for a referral to a counselor, or psychologist, or even just ask around for a cheap one, although they might be a little pricey I'm sorry.
Good luck, have a lovely day.
- Sarah