How exciting for you, or not as the case may be, I know I was terrified at my first show, and I had an 'old hand' with me, who I tried to persuade to take my dog in the ring when I felt I needed brown trousers!
Anyway, once you have done it once you will either love it, or as one of my friends said, 'hate it' as she was a nervous wreck and wouldnt do it again.
At least you wont need all the paraphernalia I have with a long haired breed, just the dog and a packed lunch. Are you going to ringcraft so you know 'how to show' your dog, and have some practice with what may be expected. Or could you go to a local companion show beforehand, just so your dog gets used to the atmosphere, If you are the first class in the ring, make sure you go last in the line, so you can watch what everyone else does, but dont worry about it, as the judge will tell you what they want, usually a triangle and up and down, and sometimes all rounder judges get you running round the ring like maniacs!
I am not familiar with how Mastiffs are shown, but I always take 'bait' to keep the dogs attention in the ring (garlic chicken or sausages).Lankeela, are Mastiffs free standing?
Enjoy......and please let us know how you get on.