Study, but don't get too worked up over the final score. Figure out what you want to do and see if you scored well enough for it (I don't know what a 51 qualifies you for, maybe a classifier will show up in here and tell us, or you can google the jobs you want and see).
You might improve your score somewhat, if you fucked it off pretty badly, but you might not -- the ASVAB is an aptitude test, not a knowledge test.
I can tell you that you won't qualify for IT with a 51. The jobs you're looking at based on what you typed here are MA (Master at Arms -- Navy police), YN (Yeoman -- administration/office), IT (Information Technician), and HM (Hospital Corpsman). You can google any of those jobs and it should tell you what the minimum ASVAB is for them.
All you need to do is 36 months for GI Bill, if you get to twelve years you're not going to get out -- 20 is just too close at that point. The only way you can do 3 years is to sign up under one of the apprenticeship programs, which I would never recommend -- if you get an A-school you'll have to do 4-6 years at a minimum.
There will be much cleaning of bathrooms, probably no cooking unless you go CS (Culinary Specialist), but until you make some rank you're probably going to do some cleaning. There's a lot of cleaning that has to get done on a ship.
If you want to make money, see if you can get on a ship overseas, it's better money there than other places.
I'm a Chief Electronics Technician, about to go over 20 this year.