SATS and all those tests are really freaking me out! I dont know where to begin!!!?

Hi, I'm Sammy and I'm in 7th grade going to eighth in a few months. I heard about the many tests which you can take in high school like SATs,ACTs,MCATs, PCATs etc. I dont know anything about these tests. What are the different tests out there and what subjects matter do they cover. Also, since I'm still in 8th gr. (almost) what ways can I prepare now so it will be easier as I get older?