when you get a critical hit, your weapon damage is a regular damage roll + the max damage die+ proficiency (ex: longsword 1d8+ str, crit longsword 1d8+8+str)
for magic attacks, if your ranged attack is a crit, you roll additional dice similar to a weapon attack. regular damage roll +the max damage die.
(ex: chromatic orb 3d8, crit would be additional 3d8+24)
for magic saves, if the target crit fails, the target gets dealt damage following the same rules above or if the spell does deal damage, the target gets disadvantage to try to break out of the spell for 1 round (edited)