What I should be wearing for the event, tips for the beginner?

Swim
Swimming costume or tri suit
Swim cap if required
Goggles

Bike
A bike - in good working order
Helmet (no helmet = no race)
Clothing for the bike or tri suit
Drink for bike

Extras...
Cycling shoes for use with clipless pedals.

Run
Running shoes 
Clothing for the run or tri suit.

Extras...
Tri suit. A suit which you can use for all elements of the race so you don't have to get changed. It can be used for the swim and has a bit of padding in the seat for the bike.

Race belt for number (or just safety pin it to your tshirt)
Towel
A smile

If you have a trisuit, wear that for the entire event. If you don't have one or don't want to fork out for one on your first tri, just wear a swimsuit in the event, then pop on some lycra shorts and a cycle/running top in transition.

You could wear cycle shorts if you tend to get a bit uncomfortable on the bike, but remember they aren't the best to run in so change to running shorts in the second transition.

I would get some tri shorts for your first one as you will always use them in the future to run in or swim in. Most women prefer to wear a bra - which means you will need to swim in your bra. This works fine.

If you are on a mountain bike get some slick road tyres for it as that will make a big difference.

Training

Make sure you do some training!

Timing

You are timed from the moment the starter tells you to go in the pool until you cross the finish line. The change from pool to bike is known as T1 (transition 1), this time is included in your swim time. The bike to run change is T2 (transition 2) and is included in your run time.