mgbrs

got myself a 2ltr ghia auto saph for work, and its been running fine, but gone for move it today, and seems not to select a gear! when you do put it into D or R you have to rev it like mad and then it will move a short distance and thats it!! i aint got a clue about autos, and wouldnt know where start as to what to look for any ideas or advice would be much appreciated! also how difficult would it be put a manual box in??
cheers in advance