I know it sounds mad, but the main thing is to try to relax! If you tense up, you tense up 'down there' as well, and that can make the process more uncomfortable.
Please don't feel embarassed, you're doing the right and sensible thing going for a smear test. The nurse who carries out the test will have plenty of experience (of women of all shapes and sizes) so will be able to help you to get comfortable. She will use a speculum so that she can see your cervix and she will be able to move that around a bit if she needs to to get into the right position. There will be some lube to help it on its way and it really isnt too uncomfortable.
I prefer to wear a skirt, because then you can hop up on the bed and leave it on, and the nurse can just move it when she does the test. If you wear trousers you'll end up naked from the waist down with a bit of paper protecting your modesty - probably not a good look on any of us ;-)
So just try to keep calm, and if you're nervous make sure the nurse knows it's your first one. I am sure she'll look after you and do all she can to put you at ease.
Good luck hun!