Lots of folks are intimidated about buying a wetsuit.  I'm going to attempt to demystify the process for you.   

First and foremost, for a wetsuit to work properly, it has to have a snug fit.  That doesn't mean it should be so tight that it's hard to move.  It just means that if there are any baggy areas, they will likely pool with any water that gets into the suit.  It's tempting to buy a suit for your kid that's a bit bigger so they can "grow into", but you'll sacrifice warmth, which is the entire reason you're buying a wetsuit!  

There are many different types of wetsuit, depending on how much coverage you want. Most are made with a synthetic rubber called "neoprene" that was invented way back in 1930 by DuPont, but wasn't used for surfing applications until the 1950s. Since then, the wetsuit has evolved dramatically.  Here are the categories you can choose from:

*Vest and Jacket-  These cover your upper body only.
*Short john-  Short legs with sleeveless upper.
*Spring suit-  Short legs with short or long arms.
*Long john-  Not commonly used, but sleeveless upper with long legs.
*Full suit-  Long legs with short or long arms

In a nutshell, there are three factors that determine how warm a wetsuit is:

1.  Thickness:
Wetsuits come in different thicknesses measured by millimeters (mm).  Most wetsuit jackets come in 1-1.5mm.  Spring suits are available typically in 2mm. Full suits come in combination thicknesses where the legs and torso will be thicker than the arms (so paddling is easier).  Common combinations are 3/2mm, 4/3mm, 5/4mm, and some even thicker.  

2.  Seam type:  
Flatlock or overlock seams are not water-tight, so they aren't ideal for a cold water wetsuit.  You will find your cheapest suits have these types of seam.  
Blindstitched seams, which are generally sealed/liquid taped and glued will provide a mostly water-tight seal and will offer exceptional warmth. 

(Flatlock)                                            (Blindstitch)                               (Liquid-taped)

3.  User tolerance:
Different people have different tolerances to cold.  Using myself as an example, I get cold easily, so I tend to require more wetsuit than most.  Some folks can handle colder water with ease, so they'll obviously be able to get away with less coverage or a thinner suit.  

Here's a VERY generalized guide for the "average" adult for which type/thickness of suit to get based on water temperature.  Keep in mind your personal tolerance to cold.  My personal tolerances are much lower, meaning I'm wearing a full suit until the water hits 70 degrees and can't comfortably shed the neoprene until the water is at least 76 degrees:

74+ Jacket or vest 1-1.5mm
70-73 Short john or short sleeve spring suit 2mm
64-70 Long sleeve spring suit 2mm
59-64 Full suit 3/2mm
52-58  Full suit 4/3mm w/booties and gloves
Below 52 Full suit 6/5 or 5/4mm w/booties, gloves and hood

There are basically two types of entry for spring suits and full suits: back zip and front zip/zipperless.  Back zip suits are generally easier to get into as the opening is larger.  Front entry suits tend to have small openings and require a bit more contortion to get into, so if you have any shoulder issues, go with a back zip. Because of the smaller opening in front entry suits, they tend to allow less water in.  


(Gyroll front entry zipperless)                            (Back zip suit)

Want your suit to last for more than one season?  A few basic tips to make your suit last longer:

1.  Always rinse your suit with fresh water after use. 
Salt water is corrosive...that's not news to anyone, and it's no different for a wetsuit. Plus, if you're the type that tends to empty your bladder inside your suit for that temporary urine euphoria, you should know that pee is just as corrosive as salt water. 

2.  Dry your suit in the shade! 
Much like salt water and pee, the sun's UV rays will break down your suit prematurely.  Hang it in the shade, and hang it like the pic below, NOT by the shoulders!  Check the Hang Pro wetsuit hanger! 

3.  Take care putting it on and taking it off.  
The new high-end suits all have superstretch neoprene and, particularly in the case of chest-zip suits, require some stretching and stressing of certain parts of the suit to get on and off.  If you're in a hurry and tweak something too much, expect seam poppage to ruin your day.  Take it slow and easy.  

4.  Get a changing mat to protect your suit while taking it off and putting it on.
This prevents your suit from getting mashed into the pavement or dirt as your removing it!