what would you get for your 18 year old girlfriend?

if ur talkin friends then say a cute necklace.or even better a best friend necklace.and if ur a gurl and she's a gurl.then u r being gay alot.

Something expensive, only so that I don't look like a cheapskate. In reality, I'm not in any position to spend a lot of money, but I would do it just to show effort and sincerity.

Of course, I've already tried this, and it didn't really bring a great result.

That depends on the relationship, what she likes, etc.