Read/watch a guide (or not, if you are interested in blind progression) and join a [Practice] Party Finder group that fits your criteria (i.e. "practice from start" or "blind"). There is also "beginner-friendly" symbol some groups may use. Most importantly, read the party description and don't join parties that don't suit you.

Once in the fight, just do your best. The average player in PF is pretty bad so you are very unlikely to waste 7 other players time.

Don't bother with old content as a first timer, it's entirely irrelevant and doesn't offer any sensible learning curve.