There are 22 players on the field for a normal offensive snap: 11 on offence and 11 on defence. You do not need to memorise all of them at once. Start with the jobs closest to the ball, then work outward.

Offence

PositionMain jobWhat to watch
QB — QuarterbackReceives the snap, distributes the ball and controls much of the pre-snap decision-making.Where are his eyes? Does he hand off, drop back or move the pocket?
RB — Running backRuns the ball, catches passes and helps in pass protection.Is he aligned beside the QB, behind him or motioned out?
WR — Wide receiverRuns routes and creates separation in the passing game.Alignment, motion and leverage against the cornerback.
TE — Tight endHybrid receiver/blocker who can change the strength of a formation.Is he attached to the line, split out wide or used in motion?
OL — Offensive lineProtects the quarterback and creates running lanes.Do they step forward aggressively or retreat into pass protection?

Defence

PositionMain jobWhat to watch
DT — Defensive tackleControls the interior and collapses the pocket.Can the guard/centre combination stop penetration?
EDGERushes from the outside and sets the edge against the run.How does the offence help the tackle?
LB — LinebackerFits the run, covers underneath space and can blitz.Does he attack downhill or drop into coverage?
CB — CornerbackPrimarily covers wide receivers.Press vs off coverage; inside vs outside leverage.
S — SafetyProtects the deep field, supports the run and disguises coverage.One high or two high before the snap — then watch the rotation.

Where should a new viewer look?

Start with the quarterback and safeties before the snap. The quarterback tells you what the offence is trying to organise; the safeties give you the fastest clue about what the defence might be doing.

Shortcut: learn five offence labels first — QB, RB, WR, TE, OL — and five defence labels — DT, EDGE, LB, CB, S. That is enough to understand most commentary immediately.