[Disclaimer: I went to my (now) brother-in-laws' grandfather's funeral two months ago. This is just a personal account on how I felt when I was there. To hear of his death was very shocking and sad.]
Wear all the black you can and do your best to look somber. You may not really have known the dearly departed, but you are there to pay your respects. Everyone's going to give you disdainful looks for being there, but hold yourself together and give them a sorrowful look back. Maybe your fear will be masked by sadness.
Go with your mom so you don't see too out of place. Let her do most of the talking - she knew him better anyways. Ignore how you will be ignored and relish saying your goodbyes in your head as mom shakes Uncle Tim's hand.
Sit in the back row. Find out it's not really the back row at all but actually the break in the aisle where the procession marches through. Awkwardly bow your head as the whole family, priests and altar boys walk by you. Wonder who designed the stupid aisles, curse them and blush while trying to keep a mournful look.