What are the basic requirements about a check-in agent of an airline?

I hate to waste a direct answer on this answer but... It heavily depends on where you live and the airport. Most airports have websites, you can check them to try to find the info you're looking for. For Google searches you can try some of the following phrases:

"What are the basic requirements for jobs at <airlines name>"
"Does <airline name> offer in-service training"
"Basic requirements for check-in agent at <airlines name>"
And so forth, sorry to not give a direct answer, but with the lack of information, you have provided it is very hard to help you. Best of luck!