what are the easiest forms and most popular forms of payment?

We currently use Paypal, which gives our customers the option to use a different credit card. We do get alot of online submissions, but we have customers who get confused by paypal's options. 

Paypal has a great level of acceptance. But it can depend on specific markets/countries. Where is your project based?

Have you tried Paypal Pro? You need a merchant account but perhaps its easier to use:

http://screencast-o-matic.com/watch/c2elo2nqxN

Stripe is also very popular and very simple to use:

http://www.jotform.com/help/190-How-to-Integrate-Stripe-with-your-Form

Let us know if you need further assistance.