The earlier we choose our career, the better it is for us.
The tense of the verbs must agree, though you could put them both in the past instead of the present.
Comparative 'earlier' pairs with comparative 'better', it can't be paired with a superlative as NaDa has it.
'It is' can't be contracted here.
Note added after reading Matthew's comment: the simple past is possible, but it has a different meaning; it is up to you to choose what you meant, not Matthew.