First of all, don't panic, you'll be great! And if not just pretend you are - fake it til you make it!
Make sure to ask lots of questions, go slow, keep checking your client is happy.
I always start by asking them if they have a look in mind. What kind of make up they normally wear if they wear any. How are they having their hair? colours etc. Sometimes they'll bring you pictures from magazines etc.
Make notes! Print off a blank face chart and scribble all over that sucka (something like this
http://makeupanarchist.com/wp-content/uploads/2012/04/closedEyeFaceChart.jpg)
Most brides want natural ish make up, unless they are someone who wears a lot of makeup normally. They want to look good in their photos.
Start with the skin and get the base right. It sometimes takes a while to pick the right foundation. Find out if they're having a spray tan too as that will affect the base!
I always do one eye fully and let them have a look. Then if it's not quite right, or they want to see it 'with that brown instead' then do the other eye and they can compare.
I always tell them to make sure they tell me if there's anything they're not happy with. The trial is the time to iron it out and you're more likely to get booked if you they know it's ok to make changes.
Also don't be disheartened if they don't love something 100% and you have to change things - most ladies probably do their makeup a certain way every day and might see it slightly different in their head.
Oh and if you can - take a portable makeup chair if you have one, save your back from bending over!
Make sure you have loads of disposable spoolies, cotton pads, makeup remover, tissues, baby wipes... etc. You'll need them.
There's probably loads of stuff - sorry this was a bit of a brain dump! But hopefully there's something useful in there.