Black or dark blue jeans are an essential because they are the everyday option for most guys. To really make them classy make sure they aren't faded or distressed and they fit just above the ankle and gently hug your legs not too tight but not too baggy either.
The essential T-shirts are the plain black and white ones. These pair well with anything and honestly are an evergreen style statement. Look at David Beckham, he just throws one of these on with his dark jeans and he's all set.