As an "adult learner" myself and someone who really didn't do well in high school, I would have to tell you DO IT! I've found college to be more intellectually stimulating than high school ever was and there are grants and loans that you can apply for which come very easily and at a very low financing rate. Certain states require different things though, so research throughly. My suggestion for you though is to go the military route and make sure that you are recruited in ILL, TX, or MASS, these states offer a free ride at any state run or land grant univeristy to veterans recruited in their states. Some other states offer grants, PA offers a 3000 a year grant to veterans for intance.
If you choose to got the military route, the DOD offers college courses while you are deployed for free and will pay for 75% of tuition including books while not. Once you get out there is also the GI Bill which you will have to pay into for the first 6 months while you are active. 100 dollors out of each pay will turn into 985 dollars a month for 36 months of school, which the Veterans Administration will pay directly to you. This does wonders for those in Ill, Tx and MASS, with the state sponsored dealies, they get paid to go to school. The best thing about the GI Bill is that you only have to serve 36 months to qualify. The reserves also offer a comparable GI Bill but is considerably less money. Check with a recuiter to get exact amounts.
Just some options for you to ponder over.