You need a multitude of vouches. Upon receiving these and becoming a trusted member of the community, you will then need to make a middlemanning thread in the middleman section. You have a long way to go before that though
Basically you can offer your MMing service regardless of your vouches - just people won't use you. You should offer some free questing or something to start up and then get start charging for it after you have gotten a few.
I would recommend doing skill training or something that shows some trust before doing so but after that is done you simple go to the middleman section and start a middleman thread Any questions feel free to pm me.
It's basically about the quality of your vouches. I started off by selling GP. But even when I had about 80 vouches, there was little demand for my service.