I use Paypal as my online sales management team because it was easy to setup an account, easy and free to install and is very low cost to me: no monthly fee, minimal charge per transaction. For instance, all of my work at Seven Scrolls sells for $200. Paypal takes 2.9% plus 30ยข per transaction for the first $3000. per month sales. Their take goes down as my sales go up. So for every Scroll I sell, they get $6.10 compared to a gallery which gets 50%. The biggest reason I use PayPal is that it is widely recognized and trusted.
Here's one of the many possible buttons you can use on your site with Paypal. You can learn all about their services by going to www.paypal.com If you decide to use them, their site will walk you through the whole process.
Or you can Google "merchant services" or "sell your stuff online" etc. to see other options.
A word about ethics here: if you sell your work online AND in galleries, I am of the opinion that you should work out something with your galleries regarding commissions. Galleries typically take 50%. That's a lot of money, but it's also regarded as a fair exchange for the incredible visibility your work will get on a busy street in Boston or Seattle (or in my case at the airport in Portland, Oregon) that it will not get in your studio in the Maine woods. Often, people will see work at a gallery then contact the artist directly thinking they will get a better price. This decision, of course, is up to you, but I believe in supporting my galleries just as they support me. So I have worked out an arrangement with each of them. For online sales that began with a gallery purchase or sighting, I pay (depending on the gallery) anywhere from 10-30% back to the gallery.