The first thing you should do is to see if your vehicle is rated to tow at all and, if so, how much. The Trailering or Trailer Towing section in your owner’s manual is the best place to start. You might have to adjust to your carmaker’s specific terminology, though.
Once you know your tow rating, you must consider your particular situation to determine your practical tow limit. That’s because published tow ratings are best-case maximums that are arrived at by assuming an unmodified, lightly-optioned tow vehicle piloted by a 150-pound driver traveling alone without luggage.
Suitable trailer hitches may or may not come standard on vehicles that are rated to tow a load. In cases where there is a tow rating but the hitch is absent, the manufacturer will almost always offer a factory-developed accessory that can be bought from the dealer.
Hitches are composed of three parts. The receiver is a structure with a square receptacle that is always affixed to the vehicle. A ball mount is meant to be plugged into and securely pinned to the receiver when it's time to tow and removed and set aside when it’s not.
It’s often impossible to weigh a trailer before you buy or rent. The most conservative approach is to go by the trailer’s GVWR (Gross Vehicle Weight Rating). This can often be found on the specifications sheet but is always stamped into a plate affixed to the trailer body. The trailer’s GVWR (not to be confused with your tow vehicle’s GVWR) represents the maximum amount the trailer should ever weigh.
In short, the trailer should be coupled to the tow vehicle before you begin loading it.
Trailers need to be properly balanced so they are stable when towed, and that comes about when the trailer tongue pushes down on the trailer’s hitch point. But the balance is in flux as you load a trailer, and it’s possible that the Tongue Weight could go momentarily negative and the tongue could quickly tip up as you load a heavy object in from the back.
Backup cameras help a lot here, but nothing beats a helpful friend spotting for you as you gain experience. Make sure the trailer wheels are chocked, the trailer tongue is raised high enough and the trailer receptacle is unlatched.
As mentioned before, a certain amount of downward pressure on the hitch ball, called tongue weight, is necessary to ensure that the trailer will tow straight and remain stable. That amount is almost always 10% of the trailer’s overall weight, though some heavier trailers require more. The tow vehicle’s rear suspension will compress a bit as it shoulders this load, but that’s expected and is accounted for in the rating. In all cases it’s not the weight itself that matters, it’s what it represents: that the trailer’s center of mass is situated ahead of its axle.
Trailers don’t always have brakes of their own. Brakes are more likely on heavier trailers, and at some point they are required by law. The carmaker may also have a tow rating cutoff point, above which they recommend trailer brakes.