GMOD Server

These questions are biased but I'll give you some advice.

For picking a gamemode there are two routes: Top 5 gamemodes like darkrp and TTT which are guaranteed to not die for a while but also are harder to get going, Unique gamemodes that people rarely play but if they take off can win big.
You may have to try different gamemodes until you find one you're good at hosting.

For a host I'd say either: NFOServers, GMCHosting, or ELHS
NFO is good in just about every way
GMCHosting I can't say much about because I've not used them but they've been accused of shady stuff
ELHS I've used and is decent but they're new in the scene and still have some hiccups to work out

Forums depend on how tech savvy you are. You could use phpBB or myBB but those are a bit outdated and require a lot of setup
Enjin is easy to use but I'm not sure if its free and how many addons are available
I've seen people use invision but its paid AFAIK.