is pay for parking a good idea?

[Dane Delatte]

I am thinking of buying a 26 unit apartment building that currently charges weekly rates ($200-$260 per week). I plan on keeping them weekly rates but I was wondering about charging for parking to use for fix ups of the outside the building and keep up of the parking lot. Also, there are occasionally vehicles parked there that are not tenants or guest of tenants. The business does not currently charge for parking, it is first come first serve when it comes to parking. Another factor is I only have 42 parking spots for a 26 unit. 

I was wondering:

Should i charge for parking and how much?

should I charge weekly or monthly for parking?

How should I keep track of vehicles that should be there and not?

How do I implement the system to tenants that are already there and did not have this parking fee when they started renting?