Righto!
Helmet - Over here at least all helmets must have the approval sticker, so they all comply. Technically they are all safe. Having said that, the more expensive ones will be made of Kevlar/Carbon fibre as opposed to fibreglass or plastic. Generally, they all differ a bit in shape, so try a lot on and go for the most comfortable. DO get a full face though. Half face helmets are for peeps that don't like their chins. And the main thing to remember is that you have to put your head in it! I have a Shoei, which tend to be on the expensive side, but it fit me the best.
Gloves - You generally need two pairs. Light pair for warm weather, thick pair for cold. Get long gloves (gauntlets) so they overlap your jacket, and have wrist velcros, otherwise if you come off, the glove can just get pulled straight off you hand. Also good is carbonfibre on the knuckles.
Boots - Full ankle boot, not low boots. Leather of course, and zip side, so they stay on. You can get lace-up, but that is soooo last century. If you wear pull-on boots with no zips/velcros they can just get pulled straight off in the event of "ejection". Seen it happen.
The rest - Traditionally, I used to always wear full leathers. Two piece. With extra padding in the usual areas (hips, elbows, knees, shoulders). Problem with leathers are they are not waterproof, so you need wet weather gear too. You also should have a jacket that zips to the pants. Why? So if you sliding down the road on your back it stops the jacket riding up and exposing your back (or front).
Nowadays the polyester jackets and pants are really good. I just got a new set to wear when I ride to work. It's a bit baggier to go over my uniform. Most will have a waterproof lining (but the outer shell will get saturated) or (like mine) the actual outer shell is waterproof. So if it starts raining during the ride no need for wets. Some of them have a zip out thermal lining too. So it's an all season solution. The body armour in the new suits is really good too. Some will only have hard plastic, but better ones have a special soft armour, so it offers cushioning as well as abrasion resistance (and it's a bit bendy which makes it comfy). I don't even know what brand mine are. But all the brands have cheaper stuff through to better stuff. Better to look at the features.
Main thing to remember is your gear needs to fit nicely. So it's not a chore to wear it.
If you wearing full gear, you can come of at a hundred miles an hour and still be OK, as long as you don't hit a stationary object. If it's too hot to wear the gear, it's too hot to ride.