so i just bought a 2002 ford explorer and its my first car but the lady i bought it off had the steering column replaced and the guy did not give her the old steering column back so i tried to put the original key in and it would not start so i dont know what to do

Sounds like you got into a real mess, do you have the keys for the replacement steering column? If you do then the security system will need to be programmed to reconise the new keys and column, the dealer or a good locksmith can do this. If you don't have the keys then you have a real problem. Now you can have a locksmith come out and pick out the key code, cut a new key and reprogram the system to match it, or have it towed to a dealer and have the same thing down there, either way is going to be expensive but the locksmith will be cheaper than the dealer and the locksmith maybe able to come to you and that will save a tow bill. Now you will still need the stock key to unlock the door and hatch unless you have the key cylinders tumblers matched to the new key, again more expense if you do that but it is not necessary. Good Luck on this one.