I'll try to keep the explanation as simple as possible, since you won't have to be worried too much about specifics on the MCAT.
You already know the function of tRNA and rRNA. Don't forget about mRNA! mRNA is the strand that is read. So the ribosomal complex will move down the mRNA and whatever amino acid is added next (brought by the tRNA) will depend on mRNA's base sequence. So, the base sequence of an mRNA is used to order and join the amino acids in a protein.
As far as P and A sites go, just remember that the first amino acid, Met, with it's tRNA will bind at THE P SITE FIRST. The next amino acid (for eg. Arg) will be added at the A site. The ribosome will then move down the mRNA strand, and the tRNA bound to Met will be released (through hydrolysis of GTP --- but you probably won't have to know that), and the amino acid (in my example Arg) which came after Met will now occupy the P site. A new amino acid (eg Leu) will be added at the A site. This process will continue until translation is stopped with a stop codon.
To really simplify it, the amino acids (EXCEPT FOR THE FIRST ONE --- Met)* will be added to the A site first, then will be shifted to the P site where its tRNA is released.
*Met will bind directly to the P site first instead, and then its tRNA will be released.
I hope this helps.
