I'm assuming your quite new to trials? You should definitely get a helmet, because if you're a beginner you'll be falling over and bailing a lot of things. I've done trials for about 2 months, and quite glad i have my helmet to be honest Take a look at this and you'll see why you should wear a helmet:
Oh and as you asked what kind to get, I recommend the Fox Flux. Quite pricey, but a really comfortable and nice looking helmet.
I recommend gloves, they don't really protect you from a fall, but protect your hands well. If you don't wear these you can get a lot of cracked skin and things.
You don't really need any other safety gear, I think.
Hope this helps.
Cheers,
Fraser