I'm taking 18 credits now and its tough, but I'm doing well. How you manage your time is all just a game, really.
If you're majoring in phyics, math, engineering, or computer science, your first two years would be basically the same at community college, so just take the reccomended courses: Gen Eds, Calculus, Physics, and Chemistry, and don't overload yourself your first year (if you are in fact going into your first year).
Spend your extra time getting A's on the courses you DO have, figuring out which science major(s) interest you the most, and having a social life.
And get to know the faculty. Connections can often be far more valuable than coursework. Believe me. If you're transferring, do it as soon as possible, as there is almost definitely 100 and 200 level courses required by the university that your community college does not offer, so look into it. For me, prereq's and such were different here so I will most likely have a 5th year because of that.