Buy a complete assembly. After using it for a while you may find a different set of features/function that may desire. Then go build one.
Good advice for a first AR. Having bought and built several over many years, a good basic AR in 5.56 from a reputable maker is always a great first step. Don’t mess with it, just shoot it.
From the builder side, one can travel in a million directions and the OP has given little info to cater a detailed response. There’s a significant learning curve in building ARs and my best advice is start absorbing how it’s done, what tools are needed, and (most important) make damn sure you understand the state and federal laws surrounding a build. There are thousands of AR parts and accessories out there, but the wrong combination can cause you to lose your gun rights for life.