I5-8400 is a good and appropriate all around gamer.
You can buy any lga11151-300 series motherboard.
Buy a 2 x 4gb DDR4 ram kit. Speed is not that important.
Your GTX980ti will need a 6 and a 8 pin aux pcie power connector.
Typically found on a 600w psu.
Look for good quality; here is a list:
https://linustechtips.com/main/topic/631048-psu-tier-list-updated/
Seasonic focus is very good, 550w is ok for them.
Put together a list of what you have and a tentative parts ordering list.