One field with multiple values? I know that what I'm asking can't be done or shouldn't be done, but I really don't know how to do it. This is the problem:

2 tables:

 - Drivers (id_driver, name, etc...) 
 - Trucks (id_truck, model, id_driver,
   etc...)


The user will fill a form with the truck's info, but a truck may have more than one driver, and thats my problem. I don't know what to do there.

Thanks in advance.
This is likely to be a many to many relationship. 

create table drivers (
  driver_id int primary key,
  ...
);

create table trucks (
  truck_id int primary key,
  ...
);

create table drivers_trucks (
  driver_id int references drivers(driver_id),
  truck_id int references trucks(truck_id),

  primary key (driver_id, truck_id)
);