Since you've never been down any "machine path" yet, don't sweat it, nor lose any sleep.
IMO, one thing you don't want to do, is learn some of these processes on somebody else's vehicle, especially an expensive one. You don't want to learn the hard way, and have regrets for a long long time, that you made a bad mistake.
There's nothing wrong with simply doing a good wash, vacuum, interior detail, clean glass in and out, and apply a coat of a sealant, or wax, and be done.
That above can easily take 8 hard hours of work, and it will show. Lots of vehicles don't need full blown chevy vettefest details to look good, and I'm sure you don't want to spend 20-30 hours of work, and see little gain.
Aunts, Uncles aren't looking to place their vehicles in some Barrett Jackson Show Auction. They will most likely be tickled pink with the work that you will do.
They'll see a clean interior, no dirts-dusts all over the dash, door panels, clean glass, a nice shine, water beading, and a paint that you know will at least be protected for a few months time.
We don't know the condition of the paint, so it might be filthy, might be swirled, might be hazy, a few light scuffs here and there, tar, and sap stuck to the paint, tires that have never been realy clean since the day of delivery.
If you're looking to buy strictly over the counter, one good bet would be meguiars products. Have an all purpose cleaner on hand, the Megs, or Mother's Clay Bar Kit, and perhaps just a simple red bottle of Meg's Cleaner Wax for $6. If you want to get fancy, then a bottle of Megs Mirror Glaze #21 Poly Sealant for about $19-$20 is another good bet, real easy to apply and take off, and usually lasts a bit longer than their waxes.
Just the few products I mention above, besides some common stuff you might have on hand, like Windex, brushes, wash towels, all purpose cleaners, detail brushes, and other small handy tools, the costs above will easily come to $50 or maybe a bit more.
A vehicle that is drivem but not taken care of like a '67 Stingray show car. gets dirty quick. Filthy Door Jambs, Trunk, under Hood, Floor Mats, and Carpeting. Glass that hasn't been touched in months. You get the picture.
Mark
Have a few applicators, and microfiber towels to remove waxes-sealants.