if i buy a car and i want to pay a monthly fee around $150, what would the down payment be?

That would depend on several factors. How much is the car that you want to purchase? How long do you want your loan to be? My wife and I purchased a car for a little over $6,000 and the type of loan actually needed to be over $6,000 to go through correctly so we were asked not to do an upfront down payment. I am guessing if you went to purchase a more expensive vehicle like over $10,000 then you would start being asked about putting $1,000-2,000 down. All of this varies between how much the car is, what loan options you receive, and how long you want to pay for your car.

Since you're looking at a used 2006 Honda Accord...I would expect the asking price to be in the $7,000 - 10,000 or 11,000 range. If your credit is good to excellent, I would say a downpayment of $1,000-$1,500 would get you that type of monthly payment (or even less) approximately.