Asking for Fundraising at work?

I am 24 years old and suffer from Rheumatoid Arthritis. I am participating in a walk on May 1st. How do I ask for donations around work? Is it appropriate? The website lets me send m e-mails, do I? Do I post a flyer somewhere? Or just leave it to my friends. I feel I could earn alot more if I let my co-workers know. Thoughts?