a little=a little bit+adj./adv./uncountable noun.例：1.I'm a little / a little bit hungry.2.He ran a little / a little bit slowly.3.He wants a little / a little bit bread.
a bit=slightly+adj.例：1.This coffee's a bit cold.2.I'm a bit tired.
a bit of+ noun.例We've got a bit of a problem with the computer.