OK. Since your destination is West 30th Street between 6th and 7th avenues, I recommend taking the AirTrain to Jamaica Station, and then boarding a Manhattan-bound Long Island Rail Road train to Penn Station, where you can exit onto 7th Avenue at 32nd or 31st streets.

From T7, follow signs to ground transportation and AirTrain. You don't pay anything upon boarding at the airport; you pay only when passing through the faregates at one of the two AirTrain terminals, Jamaica or Howard Beach. You will board a Jamaica-bound AirTrain at T7, not a "Howard Beach" or "All Terminals" AirTrain. (In this context, "All Terminals" refers to all airport terminals; that route does not go to either of the AirTrain terminals.)

Upon arrival at Jamaica, you will walk to the faregates, and will find MetroCard vending machines where you can top up your existing MetroCard to reach the $7.75 in value that is charged to pass through the faregates. Before you do so, you may also be able to purchase your Long Island Rail Road ticket from one of the other vending machines near the faregates; if not, you will find LIRR ticket machines once you walk out of the AirTrain terminal onto the LIRR mezzanine overlooking the tracks.

Because you will be traveling on a weekend, you can take advantage of the special $4.50 "City Ticket" fare offered by the LIRR. Details here:

https://new.mta.info/fares/cityticket

Manhattan-bound trains typically depart from tracks 1, 2, or 3 at Jamaica. Trip time from Jamaica to Penn Station is ~ 20 minutes. It will be a more comfortable ride than the subway, and service is frequent on weekend afternoons. You can find schedules at the new.mta.info website.