When you're starting you mostly want to build with pine since it's cheap. You can get decent wood at Lowe's and Depot. You will want to look for boards that are labeled "Select boards" or something similar. Most of the wood sold in box stores are meant to be hidden or painted. "Select boards" are the better looking boards and are appropriate for finishing with the wood grain visible. They are more expensive than general lumber, and you'll still need to poke through them to find the best. They also have a small selection of Poplar, Oak and sometimes Walnut, Cherry and Maple. It's a crapshoot from store to store what types they actually carry. The hardwood in Depot is usually along the back wall behind all of the lumber.

If you want to find pretty much any type of wood you'd like I'd recommend going out to the Frederick Lumber Company. They have good prices for hardwoods, can mill stuff for you for a price, and also have a lot of very nice common and exotic hardwoods pre-cut and milled. Don't be intimidated by going there. The people working there are extremely nice and helpful.

https://www.fredericklumber.com/

As mentioned by some other people you can really learn a lot from youtube. Steve Ramsey - Woodworking for Mere Mortals is a great place to start. He even has some paid online classes.

I actually have bought wood from both big box stores, but mostly pressure treated boards and plywood. If you pick through, you can normally find something straight and not full of knots and defects, but it can take some digging. Almost everything they offer is crazy wet still, so I often sticker it and let it dry a bit in whatever environment it's going to end up in. Hasn't caused too much warping for me yet. I've bought some poplar there, too, which was actually in really good shape, but was way overpriced.

Beyond that, I've had really good experience with John Wilson Lumber in western Howard County, and Freestate Timbers in Timonium. Freestate has a pretty crazy selection of slabs and boards, JW has more sheet goods. Both can cut things for you.

Beyond that, check craigslist and Facebook marketplace. There are often either small mill operations or people getting rid of a fallen tree or downsizing their board collection or selling leftover stuff from projects. Obviously selection can be rough, but you can get some great deals.

We’ve visited this guy, but not for a few years. Call beforehand, great selection. Guild discount.

timberline

Beyond that, woodcraft mentioned below. I know there is a store in Beltsville but the name escapes me.

HC library has a few tools you can check out, I think from the savage location.