As one of the most important area of an internet business is payment gateway. A payment gateway need to put on a sales page in order to proceed further if your visitor is planning to buy your product online.
2 famous payment gateways that I usually used were:
I would recommended any of the site using these 2 payment gateways, due to the following reasons:
1) Widely accepted
2) Easy to install
3) Can be customizes accordingly to your need
For details on how to use, please search on Google.com where a lot of learning resources will teach you how. Anyhow, the installation for both are very straight forwards.
If you have no budget, please install at least one payment gateway where I would recommended Paypal which is very well known and is free to use but they will charge you certain percentage on the transaction basis. Before you launch any of the payment gateway, please remember to test it out yourself to ensure is working.