Adding text links in addition to banners will greatly increase your click through percentages. Banner ads are becoming so common that many people don't give them a second look. Also, the placement of these banners and text links is important too. Putting these at the bottom of your pages will not result in as many hits as you would have if they were displayed in a more prominent place on your page. Most affiliate programs offered today are absolutely free to join.
All programs are different.
Some issue once a month, some issue every quarter, etc. Also, many programs set a minimum earned commission amount which an affiliate must meet or exceed prior to a check being issued. This is extremely important, as this number will tell you how much traffic you must generate before you can expect to earn a commission from a sale. You need to be confident in the program's ability to track those people your refer from your site. This is the only way you'll get credit for a sale.