1. find a rat you like. another boy or a spayed female. the younger the better but don't be put off by age, there are many older adoptables out there. try petfinder.com or a local shelter first, then a breeder, *then* a reputable petstore.
2. isolate the two rats for at least 2 weeks. the best isolation is another house, but if you can't do that, get them as far away from each other in the house and wash hands, change clothes, etc. between touching each rat.
3. let them run around together on neutral turf, like your couch or bed or bathroom for an hour or more a day for a few days.
4. you can switch rats into each other's cages so they get used to each other's smells, it really helped with my girls.
5. deep clean the cage you're going to put the new rats in, rearrange stuff if you can - this way the current resident rat can't claim territory cuz it doesn't really smell like his old home or look like it.
6. put them in the cage and watch them carefully. fighting is okay, generally it's "no blood no foul". if there is severe fighting or other problems, take them apart and try again. lengthen step 3 mostly.
any way, when you get another ratty, let us know how it goes, we can def help along the way.