I am not sure what exactly do you mean but here is a tool that might help, perhaps?

http://web.expasy.org/translate/

Since many genes code for different (spliced) transcripts, you should mention the transcript-ID (http://www.ensembl.org/Homo_sapiens/Info/Index). For describing a new variant/ mutation, you can find valuable information about nomenclature instructions on HGVS (http://www.hgvs.org/).

Another tool that might be useful:

http://web.expasy.org/protparam/

You can use UCSC genome browser : http://genome.ucsc.edu/index.html  

1. Blat your sequence or input the genomic position of your sequence under the first tab"Genomes" 

2. In the "conservation" section of the browser, it will show the actual coded amino acid sequence for your input region.

http://www.generunner.net/

From where you got the gene sequence. if it is from ensembl then it numbers the codons. all you have to do is click on the cDNA from left side menu. 

compare your mutated sequence with this cDNA sequence and see which codon is changed