the season of the 18th birthdays

This year is the the season of the 18th birthdays. All of my friends are turning 18, including me. I don't know if I should be happy about this or not. Last weekend I went to a party thrown by a classmate. Actually, I was the only girl there. But it was super fun, even though I didn't think it would be!
When I think about my birthday in december I get a little bit scared. I know I should do a party (you're turning 18 only once) but the only people I could invite are my classmates. I don't have a lot (hardly any) friends outside school, which makes me sad! But I'm not good at making new friends and I don't know what to do.
Any advice? x