Making AI not walk in walls


I am working on the MakingGamesWithBen tutorial where you use C++, SDL and OpenGL. I have made a little zombie game, the zombies walk to the humans. But if they are on another side of a wall they just walk in to it. Any tips for where I can look/ I think I should use som type of path recognition but I don’t know how? Also how hard is there to make a tilemanager for a quite beginner, and how advanced games can I make alone(I know there are a lot of different answears, can just get a example)? And at last sometimes my humans go outside of the map I have included deltaTime, is there anything more I can do?
The source code is here:

OpenGL is a rendering system. Unless you are using OpenGL to do pathfinding (and you’re not), pathfinding is off-topic for this forum.

Just Google around for pathfinding and/or search algorithms.