How do I account for a sloped floor when laying out these pantry shelves?

You need to find the highest point on that tile floor and make that your primary reference. Use a 4-foot spirit level or a laser to mark a level line across all three walls starting from that high spot. If you measure up from the floor at every corner, you're just duplicating the slant of the house, and your shelves will look crooked against the door frame. Plus, round jars and cans will eventually migrate toward the low side if the shelves aren't true.

To handle that tapering gap at the bottom, the best move is to **scribe** a base or a toe kick. You set your bottom-most piece of trim or a plywood plinth leveled up on shims just above the floor, then run a compass along the tile to transfer the floor's profile onto the wood. Cut that line with a jigsaw, and the piece will sit tight against the uneven tile while providing a perfectly level surface for the rest of the build. It's way cleaner than trying to caulk a massive gap or ignoring the slant. Since those gray walls are wonky, don't assume your plywood shelves will be perfectly square. Measure the width at the front and the back for every single shelf beacuse closets are notorious for narrowing or widening in the middle. When you're screwing your cleats into the studs, use shims if there's a big hollow in the wall. If you just crank the screws down into a bowed wall, your cleats will follow the curve and your plywood won't sit flat, wich leads to rattles and instability.

For the weight of heavy cans and appliances, 3/4-inch plywood is the way to go. If the spans are wider than 30 inches, add a small stiffener strip—basicly a 1x2 piece of solid wood—to the front edge of the plywood. It stops the shelves from sagging over time and hides the ugly plywood layers. Once you've got the first shelf leveled and scribed, the rest of the layout gets much easier to manage.

I'm super new to this and that scribe thing sounds realy cool but also kind of intimidating. How do you actualy use the compass to mark the wood without it slipping? Also, if the walls are as wonky as the OP says, do I need to worry about the shelves being wider in the back than the front? I'm worried if I cut them all the same size they won't even fit or there'll be huge gaps on the sides. Is there a trick to measuring those weird wall angles?

Don't overthink the scribing. Grab a General Tools 842 or a standard Staedtler compass. Set the gap to your widest floor deviation, lock it, and keep the point on the tile while the pencil marks your plinth. As for the walls, never pre-cut all your shelves. Use a Bosch GLM165-25G to get exact front and back widths for every single level. If you're using 23/32" birch ply, you'll want some Titebond II on those cleats before driving 2-1/2" GRK R4 screws into the studs. If the wall is bowed, don't force the cleat—back it with a shim so your shelf stays dead flat.

Quick update—I grabbed a laser level like you suggested and found the high spot, wich made a huge difference. However, while trying to scribe the bottom plinth to hide that floor dip, I noticed the back wall has a massive bow in the middle. Should I shim the cleats out to keep the shelves straight, or will that leave a weird gap behind the plywood?