Squeaky rear wheel when rotating

In my experience, that sound is caused, not by the hub, but because the tire is rubbing against the brake pads. This can happen if the tire is slightly out of alignment or isn't properly trued, or if the brakes got accidentally bumped while riding (depending on your type of brakes). Try spinning your tire and watching it move between the brake pads, and you should see some wobble to it.

If this is the case, try removing the wheel and putting it back on (spin it again before tightening it in, and adjust it accordingly). If that doesn't do enough, try adjusting the brake pads (a different procedure depending on what type of brakes you have, so I can't really elaborate). If it's still giving you trouble, then the wheel is misshapen, and needs to be re-trued, which is probably best done by a bike shop, but isn't too hard to figure out on your own.

It sounds to me like your bearing cones are way too tight, or maybe not well lubricated. I hope you didn't spray WD40 directly into the hub -- that will dissolve the grease and dry out the bearings.

Take the wheel off your bike, hold it by the axle and try to spin it. Does it still make the sound? If not, you're lucky and it's probably the brake pads like @Gwen said and you can ignore the rest of this. If it's still noisy, does it go for several revolutions on its own or does it stop after a half turn?

Over-tightened cones can be adjusted and re-greased, but if you've been riding it like this for a while the bearings will be a mixture of fine steel powder and lumpy rocks. As you spin the axle, if you feel places where the axle grabs or grinds, you probably need the bearings replaced, and maybe the cones if they're really worn. Worst case, running too long with over-tightened cones can wear out the cup in the hub itself - and it's cheaper to replace the wheel. But that's pretty rare in my experience, and your bike looks new so it's unlikely.

Servicing a hub is not the best starting point for a beginning mechanic, but there are several good tutorials online. If you're handy and have access to cone wrenches it's pretty straightforward and fun.