Before you get your guinea pig you have to make sure that you have a hutch and everything else you need for a guinea pig.
When you are looking for a guinea pig it should preferably be young. The younger the guinea pig the easier it is too tame. You can get older guinea pigs which can be tamed but will take much more time and effort.
The guinea pig needs to be clean, tame, active and have clean hair. This is a sign that the guinea pig is healthy.
There are many different guinea pig breeds check out this article to help you find the right one for you.