Rifts is a world created by the GM or game master.
The first thing to know about GMing is who the characters are. This means you need to know all their skills, M.D.C., S.D.C., attributes, equipment, etc. A good way to keep track of these is to have your player(PC's) make their characters and when they are done, copy the info onto another sheet for you.
Second, the GM is the God of the game. Without a GM a game cannot exist.
It is alright to have more than one gm, but some rules of that are:
1. Never use another GM's NPC's(non-player characters) because you probally are never going to use them correctly.
2. Don't take over a campaign that someone else started unless you are just following a detailed plan approved by the campaign creator.
3. Do not purposely try to muck up another GM's campaign. This can lead to loss of trust, respect, friends, and compassion in the future because some campaigns take weeks, months, or sometimes years to do. If you screw it up by taking it over, kill several vital characters, or change the plot, there's one thing you can do: Run. A campaign can take a long time to
think up and figure out what's going to happen in it. If you ruin that you expect never to be allowed to GM again.

2. Make bad things happen to their characters. Curses, diseases, dismemberment, what ever works best to get them back in line.
3. Kill their character(this is a last resort because outright slaughter of a character is bad because it may drive away the person playing it).
4. A truly last attempt thing to do, is say, "forget you guys" and walk off and find a new group to start anew with.

Never forget that the players are the ones that are technically "writing the story," so if you write a story that has to be followed to the last detail by the characters it will fail. The players can't read your mind (please no arguments) and they don't think exactly like you. If you write a story and expect the players to follow it, you will be extremely disappointed.
2. Making replicas of items you have the characters find can sometimes be fun if it's hard to describe or it's a weapon and you need a quick live action mock battle to make clear what the characters are doing.
3. If you do have a mock battle, never have body contact. At some point, someone will accidently do something too hard, and it may seriously hurt or piss off who ever it was accidently done to.
4. Always write out details about the adventure the characters are on. Don't be so detailed as to say if the characters don't do this, they will mess up everything, because it's no fun. Be detailed enough so your not thinking to yourself for too long about what's inside a storage locker, or on how much MDC a guard has.
5. Breaks during a game are essential to keep your sanity. Many games can go from about three hours to a week, so take frequent breaks otherwise some of the people in your group will suddenly think they are their character, and run off into the forest looking for treasure. Not that I've done anything like that...
6. The location of where everyone is going to game at is important. If you make the game forty miles from everybody and tell them to be there in ten minutes, you aren't being fair.
Try and make it the same distance from everybody (smack in the middle) and make sure you tell everyone where it's at so they don't go looking for everyone in the wrong spot.