See your GP to find out if you are definitely pregnant .
If you are and you want the baby , then go ahead and have it (unless your GP advises against it for medical reasons) . I'm sure your boyfriend will change his mind once he's holding his own child in his arms . Avoid mentioning that your boyfriend is 18 because strictly speaking he (and you ) are breaking the law .
There are a number of sites on the internet that offer support for teenage mums - I did a search and this is one of many that came up :
http://www.bubbalicious.co.uk/
I'm sure that friends , family and your local church (vicar or priest) would offer you support . Best of luck - I hope it goes well for you .