LAUGH! Just kidding, no laughing will go on here, we are here to help.
An 1800 is 600/600/600, which is a respectable score, and if you work hard enough you can do it.
Prep book will cost FAR less than a class, but nothing short of studying on your own VERY hard will get you from a 1300 to an 1800.
First, take a full length practice test again, and grade it as the SAT would. Hopefully that will give you a better standard, as the one time you took it could have been a fluke.
Second, learn the SAT! There is no penalty for guessing on the fill-ins, you lose 1/4th a point for every wrong answer. Know how many you need to get right to hit a 600 in each section, and DO BETTER than that.
Figure out your weak areas, and study them specifically!
At the beginning, take tests with NO time limit, open book, and LEARN the ones you get wrong.
Good luck! And what books have you been using?