Welcome to the hardest part of your life. I always thought that the insecurities that coincide with being a high school student were rought, but the first year of college is the point in your life where you make your transition into the real world. I had a really hard time myself. I went there not knowing anyone and ultimately isolated myself for the first semester until I started to run into those that I went to high school with. One thing that I learned is that there are many others in the same ship as you. They are just as timid and fearful. Whether you realize it or not, those in college are not like those in high school: They are not out to take their insecurities out on you. They want to be your friend. There are many ways to meet others and rid yourself of the isolation. What I did is highly unsuggestable, and that is drinking. Drinking is rarely safe unless done in moderation but makes one a social butterfly. If you think that it's a possibility that you may have an addictive personality, you should probably not do that. I am reitorating that I am not recommending that you start doing that. It may sound very lame but joining a sorority would help you make instant friends. Even pledging you would meet others in your shoes. There are also many different clubs at your school that you could try to get involved with. No matter what, it will take some motivation to get out there and make these changes. It is totally doable. As hard as it is, try to lift your head up high as it has been down for so long, suck up your pride and make the appropriate changes.
As far as the grades go, talke to your instructors and see what you can do to improve your grades. You are capable of getting good grades. You were accepted into a college! It's not too late to turn things around. You have what, a month or so left in the semester? It would probably be worth the extra work to not flunk out and have to start all over again.
You really can do it!