Hi Sophie,
It depends on which test you had done. Assuming it was the autosomal one ("Family Finder", you go to gedmatch.com and follow the instructions there after having downloaded your raw data from FTDNA (the company that has your results for the test you purchased through Geni)
I wrote a post on how to do this with the data from 23andme or FTDNA here:
http://www.mylinktothepast.com/2011/11/gedmatch.html
It makes no difference if you ordered the Family Finder test through Geni, the raw data is the same at FTDNA. Follow the gedmatch directions for FTDNA, as they are the company that does the test, and where you access your results. The results are exactly the same as if you had bought directly from FTDNA.
Gedmatch is for autosomal DNA, the other sites mentioned by Harvey are good for the other tests if that is what you have (mtDNA or Y-DNA)