You need to try a few models, ideally with your glasses on to get one which comfortably fits your head.
Helmets protect you from abrasion and also reduce the forces from a sudden impact on your brain by decelerating your skull over a short distance as the padding and helmet compress. The latest marketing thing is to add a layer which absorbs shearing impacts (e.g. latest scott helmets). However, certification has not caught up with this to measure the effect, and it's not very clear whether helmets really need this given they shift on your head on impact anyway. The top of the range helmets are definitely better in terms of venting, quality of padding and chicness etc, but in terms of protection there appears to be little difference between a $300 helmet and a $50 helmet if both are certified to the same standards. Certainly when it comes to cycling helmets, the more expensive ones are designed to maximise airflow and minimise weight while still meeting a standard, so can be worse from a protection point of view than cheaper, thicker heavier helmets which have more depth of material to decelerate your skull on impact. Cycling studies also show that older EPS helmets work as well as new ones, so no problem buying last season's colour.
So really you need to make 4 choices on what to buy:
1) How many impacts does the helmet need to deal with? Either you can get one constructed like a skate helmet to take multiple impacts - typically constructed with a polypropylene outer and open cell foam, or a one-hit helmet either constructed like a bike helmet from EPS with internal plastic ribs to hold it together, or a motorbike type helmet which is similar but with a glass fiber outer layer.
2) What style do I want - race look, recreational ski look, skate look, mountaineering look, jet pilot look...
3) Does it fit my head / glasses?
4) Do I like the brand / style / colour? e.g. Giro (fashionable mainstream), POC (Crazy Swede), Scott (off piste shredder) Casco (Italian jet pilot), Shred (Liegety lover), Unbranded (Too cool to care).
Then you need to choose were to buy, either at the store who let you try on all their lids with your glasses, the internet, big box retailer... Typically there are some really good bargains around - my wife's top of the range Giro helmet was GBP30 from the UK as it was 2 seasons out of style instead of a crazy GBP150. Currently the best value helmet in Switzerland is from Landi, a kind of farmer's store, who is selling something similar to a Giro but with worse adjustment system but better goggle strap in matt black only for CHF39. Probably not a good idea to buy a one-hit helmet second hand. If it's at all cracked or compressed it's already had its one hit. And you often can't see cracks until you take out all the padding and flex the helmet.