Basically, miners will be rewarded solely with the transaction fees.

Some more information on this topic:
https://99bitcoins.com/what-will-happen-when-all-bitcoins-are-mined/
https://news.bitcoin.com/what-happens-bitcoin-miners-all-coins-mined/
https://bitcoin.stackexchange.com/questions/8439/why-was-21-million-picked-as-the-number-of-bitcoins-to-be-created