Tabletop RPG systems besides D&D?

hmmm, I actually only ever played DnD and pathfinder. 5th edition DnD is the least complicated with the fewest and most logical rules. From my own experience with RPG, you'll always have a lot of rules and it takes time and effort to master it. Though the dm is free to change a few things.

As for dm'ing, depending on what kind of campaign you want to run the workload changes. If you work with people who like to roleplay, the thing you could start with first is the general storyline. If you frow a handfull of low-level monsters at them during the first few sessions (like rats) you should have time enough to figure out how things work and what playstyle you prefer. In these cases, most of the story is decided by your players' actions.
If they're more dungeon crawl oriented (fight, kill, loot, repeat… maybe some exploring and investigating) you could look up some old dnd maps on the internet and then flip through Xanathars guide and the dm guide for some fun traps. For monsters, you can just prepare a bunch, keep the stats with you and throw them at your players as needed. In this case, have some monsters bound to specific locations/ conditions and some for randomness. If you're worried about the amount you can throw at them, use this: http://kobold.club/fight/#/encounter-builder
It gives an indication of difficulty though more monsters means more trouble for you and the players in general because there is more to manage. Also, if you're just starting out, try to avoid enemies or NPC's that use spells as it gives to much work when playing as a dm fo the first time.

Actually, I'm just going to give you the link to my step by step campaign build template: https://mwpoel.blogspot.com/2018/10/how-to-make-campaign-in-6-steps.html

Good luck with DM'ing!