So to kick this off, hosting a for relay/exit node use alot of bandwidth. When I say alot of bandwidth I mean I have heard of many reaching into the TB range monthly. Not alot of ISPs( internet service provider) support that kinda stuff happening on their Network. Tor has a list for you in order to find a good ISP here. If you don't want to do all hat you can host one with DigitalOcean and pay $5 for a nice droplet.
Instructions can be found with a quick Google search, Tor has alot of information for you to use also in it's Documentation.
Running an exit node also will get the attention of companys, alot of the time you will get emails about copy right infringement and all that because people going through your node are doing that. It is recommended that you reply to them. All the information about legal issues and how to deal with a use emails can be found here