You would want to take a Y-DNA STR test for genealogical purposes and to get your feet wet. You would want to test where the project for your surname is located. The genealogical application depends on comparing results to others with the same or variant surname.
There is a small Rainer Project at FTDNA with 2 members. It is probably too small to really tell you much, until you recruit more people. You could post messages on the Genforum surname board for Rainer and variants.
You will want to order through the project, since you get a small discount: