There are 2 types of Juggernaut.
1- Is the Assault killstreak, which is 15 kills without dying. For that, you don't have to use a riot shield, I'm assuming you can use any gun, but it's much harder to earn.
2- The Support juggernaut, which is an 18 point streak (both kill and objectives count). It comes with a riot shield and a mini-radar around it. One of the IW developers said that your team should move with whoever is the recon Juggernaut and provide him with support and stuff