I generally find myself wearing a collared shirt. For dress, I'll wear just a regular buttoned-up and collared white shirt, but I've also got a black polo style shirt with only the two buttons up top that I wear often. Sometimes, I just wear a t-shirt. It really depends on what you'll be doing.

Those are just some suggestions there. I'd also like to reiterate what Magnus said about patterned shirts. Solid colors (that match your tartan, if it's a traditional) work best.