Most companies supplying RNAi (eg. Dharmacon, Qiagen, Invitrogen) will have the software for designing custom siRNAs available on their websites. They will also provide predesigned sequences for the number of genes so if your gene of interest is among them you can test these siRNAs. General rules regarding designing siRNA sequences and RNAi based experiments can be found online (e.g. http://www.rnaiweb.com/RNAi/siRNA_Design/). Good luck