Getting an online merchant account isn't really that difficult but I found there were a lot of hoops to jump through. I went through this about 3 years ago - so things may be somewhat different now.
I use a company called Itransact but another good one is Moneris
A word of caution is that there is a monthly charge of about $10 and a minimum charge for both Visa and MC. So be sure that you are already making money before investing in this or you will be loosing money!!
The ITransact option works through TD bank, if you want to have it through another bank you will pay an extra $10/month (approx).
Hope this helps.