One of the ancient standbys, the original 'Food Fight' out of first edition, is a good one to use to get used to how the game is played; I believe there's a 5e version, though I haven't ever played it myself, either as GM or player, so I can't give you a blanket suggestion offhand. You might snag the first mission in the current timeline (Chicago) to run them through; I don't believe it's too terribly insane. One that would IMO be best would be the 'Walk in the Park' from the first Missions set (and it's free), but you'd have to convert it, and since you're just learning, that's likely to be a bit more than you're ready for.
In regards to minis, I admit to coming from HERO system in that regards, which uses hex maps/facings, and therefore IMO handles both straight and angled movement pretty darn well. Were I to play and use a map, I'd use the hex map and set each at a standard of 2m - most people are going to have roughly that amount of space for combat movement, and if they're crowding into one hex, they're likely either packed behind cover or else in grappling mode. If you use 5m, you are very, very likely to wind up with everyone in the same hex - remember, 5m is over 16', 8' in every direction from the center - you can cram a lot of people in there. 2m is only 6.5', and the next hex over is roughly within arm's reach.