Things to consider
Weight
This is the weight of your boat (dry weight) plus anything that is on the boat, including the motor, fuel, oil, batteries, equipment, ...
Bunks or rollers
Bunks offer the best support and a simple, long-lasting setup. Rollers allow launching at shallow ramps and avoid submerging the trailer.
Galvanized or aluminum
The frame of the trailer is usually aluminum or galvanized steel. Aluminum is lighter and offer great corrosion resistance, which means the trailer will last much longer, especially in salt water.
Tow vehicle
Make sure your vehicle has a sufficient towing capacity and the right hitch capacity and ball size.
Leaf springs or torsion axles
Torsion axles are smoother and less prone to corrosion.
Leaf springs are easy to replace and allow loads to be distributed evenly across all axles.
And more...
For more information on how to choose your trailer, head over to Venture Trailers website and check out their Buying Guide.