Your tie should always be darker than your dress shirt.
Always go with the classic windsor knot for your tie, but use the size of your head to determine whether you should go half (small head) or full (big head) windsor.
The width of the tie should match the width of the lapel. In general, thin lapels are more modern. Wide lapels are more old-school.
Your tie should JUST reach the waistband of your trousers, or be slightly shorter.