look at your tow vehicle. Get ALL the specs from the plate on your vehicle and write them down. Don't go by what the sales person claims it can tow, look at your own vehicles abilities.
^^^^^^^ AGREE with this 100% ^^^^^^^
I will also say that you need to educate yourself on the different weights and what they mean.
Dry weight, unladen weigh, Tare weight - Typically the trailer with the base options. May or may not include Propane weight. Options are usually extra weight. Also does not include Water in this weight.
Gross Vehicle Weight Rating (GVWR), Gross Weight is the MAXIMUM that the trailer can tow with all equipment, options, fluids, etc....
Net Weight, Cargo Weight is the weight of what you can add. The GVWR - the Dry Weight. (including passengers where applicable)
Tongue/hitch Weight - The weight that is effectively applied to the hitch. This is a factor of "Loading" as you can effect the tongue weight by shifting cargo in the front or rear of the trailers axles.
Axle Weight - The total weight that each axle can bare.
Pin Weight - Weight of the Pin for the "Fifth Wheel" type hitches. Typically over top of the Tow Vehicles Rear axle.
Gross combined weight Rating (GCWR) - The maximum combined weight of Tow Vehicle with the Towed trailer.
These are just a few. You need to be careful not to exceed ANY of them as exceeding them can impact your vehicles (either towed or towing) performance and/or safety. Many of the above weights apply to both the trailer AND the vehicle (Both will have a GVWR for example) and some apply to both but have a different meaning. The Tongue weight for example, on the trailer it's how much it will weigh, but on the tow vehicle (and hitch) it's how much it can handle. So if you have a trailer with a tongue weight of 700# and a hitch rated for 800# you are good, but if it's reverse... Then it's a bigger issue.
It's also good to try and keep about 80% under your rated towing capacity. If you vehicle can tow 10,000# you should try and limit it to about 8,000# for better performance. The closer you get to rated weights, the more issues you may encounter and the more difficult it will be to tow.
This is a lot of information and I know it can be overwhelming, but if you let the Salesman talk you into something, you may have bigger expenses then you expected. It's much better to know the basics before you decide to buy.
Good luck and I hope I (we) didn't scare you away. Of course if you have any questions please post them.