First of all BREATHE =)
second, really really work on your study habits. It's tough, but you're not going to do better if you havent studdied lots. Sometimes studying with a friend or somone who you can discuss the material with helps tons on retention. You could also find someone to tutor you
Make sure to get a full night's sleep and a very good breakfast. We're talking protien and carbs. No sugary cereal. Good food in the morning really really helps.
If you get stuck on a question, dont freeze on that question, go onto the next question and come back to the one that gave you trouble last.
Some people find sucking on a hard candy like a Jolly Rancher helps them concentrate.
and like I said before BREATHE. You'll do fine. No need to panic.