Don't be so insecure, I'm 18 and haven't had a girlfriend, too much hassle at this age in my opinion, I'd prefer to be single and have more fun than worry about responsability. Im not saying you should be the same but dont feel under so much pressure to get a girlfriend.
Having said that, you probably have a few things to improve. Generally it is upto the guy to get a girlfriend although some girls take the initiative, but you should be generating interest at least, keep yourself tidy and stuff, be outgoing and have a good sense of humor, all the typical stuff and you'll be fine..
Id just say don't pressure yourself into feeling you have to have one, and always remember there are MORE FISH IN THE SEA, there are 3 billion females out there, don't kid yourself into thinking only one is the perfect one, especially is that one has since got a boyfriend.