Counseling may be advised. Maybe for you, as well. Let your dog know that you'll love him as much as ever, unless of course you got him or her for the sole purpose of honing your cloning skills. In which case, fake it. Collect a cell.
Cells are small.
You can collect more than one. You'll do best, especially if you are just starting out, by finding cells which are alive. Thus, old hair and toenail clippings are really not recommended for the beginner. Have your vet draw some blood next time the pup is in for a checkup. Or, save the stuff that's left behind when you dock the tail or ears.