How do you know whether to use tá me or táim?
They are semantically equivalent; as far as I know in Connacht and Ulster you'll probably hear "tá mé", in Munster and standard Irish "táim".
Both are allowed in standard Irish! They mean the same thing. The same applies with Táimid and Tá muid.
you can use both! my advice is to use whatever is natural to me. Most of the time i use 'tá mé' but sometimes i say 'táim' and that's ok too :)
Go raibh maith agaibh!
go raibh maith agat