Most of the trails that are part of the Gunpowder Falls are reasonably beginner friendly.

The Hereford Zone trails are probably the easiest to get to, head right up 83, most have a decent bit of parking that only really fills up on the nicer days in the summer.

One recommendation is to avoid the Gunpowder SOUTH (white blazes) trail on the east side of 83 following the river that connects the Panther blue trail and the Sandy Lane pink trail. It has some washouts that become tricky to navigate.

The Central Sweathouse area is also reasonably easy with minimal ascents, though the Sweathouse yellow trail has some spots that aren't super well marked.

Central Pleasantville area, taking the Little Gunpowder white trail and connecting back to the Railway yellow is fairly easy.

Oregon Ridge has some fairly easy and extensive trails, but parking can fill up quickly on nice days.

If you want to go further out, Susquehanna State Park has some nicer trails, but some have a lot more up and down.

As for boots / shoes, it's best you go to REI or w/e and try a bunch on. I am personally partial to Keens, as I have somewhat boxy feet, and I like the more squared off toe box. And they've done right by me twice now with warranty claims.