Hand of Agony, Ebon Onslaught, The Rotgarde will be worth checking out - I’m sure there are other guilds which I’m not familiar with or don’t have a strong presence on the forums. Fealty to the Grave, which is Death Knight oriented, exists, but I’m not certain if it’s still active.
Much of it depends on the character you’ve built, but these are worth looking at if you’re getting a feel for it - if your character has any loyalties to the Ebon Blade, how loyal they are to Sylvanas, and so on. That’ll inevitably shape the kind of place you end up in.
There’s a number of guilds which accept forsaken but aren’t forsaken exclusive.
I’m sure if you leave any questions or points you need help with in particular in this thread, people will be happy to help you out.