@1-800-jeepin
Is the plaid mostly navy blue and black?? If so, I'd go with a white/grey shirt and black shoes with a white accent (think classic Vans high tops). If it's cold, pair it with a black jacket.
If the pants are navy blue and white plaid do either:
any black shirt
or a navy blue (unpatterned!) shirt, but if you go with navy blue, make the shirt short sleeved or roll the sleeves up and stick with a black shoe. Add a black jacket if it's cold.