Just like Raptureready said:
12 1/2 inch is the unfinished size(includes seam allowance) so the pattern will say it is a 12 inch block.
Quilterscacache has a great selection of easy blocks. Sometimes the simpler blocks are the most striking and fun to make.
Accurate 1/4" seams are important! Lots of help here on the board to figure out if your machine is marked correctly and how to make sure you have accurate seams. Being off "just a smidge" will make your blocks come out uneven or the wrong size.
Practice, Practice! Have fun!