Gene Ontology is a really nice idea! Below you can find an initial reference to it.
GOstat: find statistically overrepresented Gene Ontologies within a group of genes. [Bioinformatics. 2004]
This is their web-site:
You will find many others on this site, and among them there is the reference already provided by Dan Gaston.
You can also try pFam for many different options:
As well as InterPro, InterProScan sequence search:
http://www.ebi.ac.uk/interpro/search/sequence-search
Have I understood it correctly and you have initally some gene sequences in nucleotides?
In this case you will need to translate them first. I'm not sure I know a lot of tools
starting from sequence in nucleotides and finishing with the protein annotation.
Only this one works like that: http://geneontology.org/.
If you prefer something different, you will need some translation tool.
There are a lot of translation tools, you will find them here:
https://www.google.ru/#newwindow=1&q=gene+translation+tool
Good luck!