"I did a lot of preparation" is correct. "A lot of" would usually be followed by an uncountable or mass noun. "He makes a lot of money," "We got a lot of snow last winter." "Preparation" can be uncountable, referring to everything you did as if it were one big pile of sand. Or it can be countable: "I made all the preparations for the trip: I bought the tickets, I made hotel reservations, and I renewed my passport." You can say "I made preparations." To communicate the idea of "a lot," you could say "I made extensive preparations."

'I did a lot of preparation.' Or 'I made a lot of preparations' 'I did a lot of preparations' sounds wrong, but I don't know exactly why grammatically.