How to choose an aviation headset?
The two main purposes of an aviation headset are making the cabin quieter, and making it easier to talk to ATC while keeping your hands free, so in selecting a headset you want to find one that will do both of those things very well.  

To figure that out there are three major things you'll want to look at when picking a headset:


Noise Reduction Rating (NRR)
This value, expressed in decibels (dB), indicates how good the headset is at preventing cabin noise from reaching your ears.  A higher noise reduction rating cuts out more engine and wind noise (which prevents hearing damage and noise fatigue) and makes it easier to hear radio calls.
Higher values are better - a headset with a 30dB NRR is quieter than one with a 24dB NRR.


  Noise reduction ratings may be expressed as a positive or negative number (it's attenuation, so negative is "more correct"). 24dB or -24dB means the same thing when reading specs.

Headset Style
There are three basic styles of headsets, and one type will probably most comfortable for you.
Earcup headsets have earcups that surrond your ear, like classic David Clarks.
In-Ear headsets have earbuds/eartips that fit into your ear canal.
Over-the-Ear headsets are like headphones and sit "over" your ear (mainly for quiet cockpits).
  
Weight
Self-Explanatory: You're going to have this thing on your head for the duration of your flight. Wearing a 20-ounce (0.6 kg) headset is more fatiguing than a 10-ounce (0.3 kg) one, and you might have some neck pain if the headset is too heavy.


  Earcup headsets tend to be the heaviest - 12 to 24 ounces. Over-the-Ear headsets usually weigh in at 3-4 ounces, and in-ear headsets are generally 3 ounces or less.



Secondary things to consider include:


Do you prefer Active or Passive noise reduction?
Active Noise Reduction can make a very light headset extremely quiet, but requires power (battery or ships power) and cost a bit more.
What accessory functions would you like?
There are headsets with Bluetooth, headsets with music inputs, etc.




Ultimately the best way to pick a headset is to try several to see which you like best. You're evaluating them on some objective things ("How quiet is it?") but also a lot of personal/subjective things: "Is it too heavy?", "Do the earcups/eartips fit my ear well?", "Is the clamping force like having my head in a vise?", etc.

Your flight school may sell headsets, in which case ask to try a few different ones on your next few lessons to see which you like best. You can also chat with other pilots and try to get a ride with them to try out their headset (the exception to this often being in-ear headsets -- some folks get grossed out at the idea of having someone else's ear gunk on their tips).