Start with a few questions.
What will the rifle be used for? IE.. Varmit hunting, long range competition, personal/home defense, plinking.
What Caliber? 223, 223 Wylde, 5.56, 300 BLK OUT to name a few.
What barrel length and twist rate? See the first two questions.
What furniture? Handguard, stock.
Personally the rest of the parts are trivial well except for the trigger. Do you want a military style trigger? ALG ACT. Or something nice like a Geissele SSA-E?
Once you pick the trigger save some money and buy it with a lower parts kit (LPK).
Now you will need a Bolt Carrier Group (BCG). PSA offers one for under $100 that is Milspec and very rugged. There's no need to spend more on a fancy finish.