You can find explanations of how to use isn't/aren't (present) and wasn't/weren't (past) in any elementary grammar book. I'm sure you don't need to be told how to use these forms.
Don't use 'ain't'. It is a non-standard form, used to imitate the speech of uneducated people. You may see it in song lyrics or film subtitles, but you should not use it yourself.
Like many non-standard features of language, such as slang and dialect, people use it to show that they are part of a group. A British or American teenager talking to his teacher may use the standard English 'He isn't here', but when he is out on the streets with his group of friends, he may use the deliberately incorrect 'He ain't here'.
I don't belong to their 'group' so it would be very strange for me to say 'ain't'. It would also seem strange, for you, as a non-native speaker, to use this form.