It's quite simple really. A = 4.0 B = 3.0 C = 2.0 etc...
A grade received in a course with more credit hours carries more weight than the classes with less credit hours (ex: an A in 4 credit microbio is more heavily weighted toward your GPA than an A in a 3 credit bio). The more credits you take, the less significant of a change in your GPA you will see (it's harder to skew the GPA with 70 credits as opposed to 30 credits), this is based off simple averages. You can calculate your GPA using online GPA calcs like this one: http://www.back2college.com/gpa.htm . Also, I'm sure you're curious about your GPA but for the time being it's just important to simply get as many A's as possible, don't worry about what the final outcome might be.
It depends on how many courses you take and what your grades actually are. For example, I got a B- my first year and that dropped my first-year GPA below a 3.5. By the end of my undergraduate career, calculating my GPA with and without that B- only changed my GPA by < 0.02 points, so not a big deal at all.
... I'm a little concerned that you couldn't figure this out on your own.
(Hint: If you want to see how a new grade will affect your current GPA, just put your total credit hours in thus far with the approximate cumulative GPA for slot 1, then put the new class + grade in for slot 2)