What type of block should I use for a small retaining wall?
It really depends mostly on the look you are going for, any kind of retaining wall blocks should work fine.  The most important thing is making sure that you put down a good foundation for the bottom row of blocks.  

Make sure to trench it out a bit, put some gravel in the bottom of the trench and sand on top of that for good drainage and to prevent frost heave (if you have frost in your area).  Level it out nice and put down the first row of blocks.  Once you have that down, the rest is easy.