You can start by searching gRNAs on your target gene through online databases such as CRISPRScan and chopchop.

Hi,

For the generation of Knock-outs, it is recommended to use 2 or 3 sgRNAs simultaneously to generate large deletions of your target locus. By using 1 sgRNA you rely on random INDELS formation which can result in on-target mRNA misregulation, leading to aberrant protein products.

Hope that helps.

Article CRISPR-Cas9-based mutagenesis frequently provokes on-target ...

http://www.e-crisp.org/E-CRISP/

This one worked very well for me.