I need your help to make my visit of Malta as low cost as possible.

Hi Peter and welcome to expat.com,

If you are looking to bring your own tent you could look at the website below. This is charged at a minimum of €15 per night based on a minimum of 2 people and also includes use of the shower.

To do any freecamping you would need to obtain a permit from local council.

http://www.maltacampsite.com/rent_a_pitch.asp

In terms of car rental, there are many places to rent a car including maltacar.com which starts at €5 per day.

Hope this helps

Jamie

Nice one thanks a lot!

Malta might not be the right place if you want to obey all rules 

You'll find a lot of campers with tents all over Malta (maybe not already in April, but later on).
In April you should also be lucky in finding rooms for 10-20 Euros per night via Air B&B (although maybe not in the center). If it's only for "now and then", entry fee to a swimming pool with shower facilities is even cheaper 
Cheapest car rental company here imho is Goldcar, however small local rentals might be even cheaper (10-15 Euro per day) since they offer older cars.