Hey mate
When I was 18, or a little less, I did my Internet research and found heaps of information on jelqing. Over two years I gained 2" of length and passed 7" girth! Gained stacks of flaccid (show off in locker room size) and had rock hard boners. I'm sure jelqing properly assisted natural development, following strict warm up/warm down routines and combining several techniques with kegels.
My advise is read up, play it safe, any pain stop and take a few days off, it's your cock.. You only get one of them!!
People may disagree but I'm going to say at your age if you can't afford a quality pump with gauge, stick to the jelqing, in my opinion it has better gains. I believe pumping has a few hours of noticeable gain and regulars say over the years they have permanent gains. From my experience a well timed pump session resulted in about an hour of boner gain and about a week of noticeably plumper flaccid size. If you do it, take extreme care, the potential like always is there to do extreme damage... Don't overdo it for short term gains.
On rare occasion I use a vacutech thick wall tube with one of their premium pumps with guages. I've read good things about the bathmate products but not sure if they are as safe. So basically, read up, manufacturers websites should give detailed info on measuring up. Also at 18 there may still be 5+ years of natural growth left!
Lots of info on this forum from lots of guys that take PE very seriously, so read up and don't rush towards fast gains, it will take time