For some reason when I have a regent, I am not able to declare war, or do some other tasks. It says it "needs to be approved by regent." How do I go about getting regent approval?
You have a regent because you are not currently capable of leading due to being too young, too injured, too imprisoned, or on a pilgrimage. If it's the latter three you'll be able to see the "incapable" trait, "imprisoned" status, or "on a pilgrimage" status on your character sheet.
Regents are more likely to go along with your plans if they have a higher opinion of you, so you can try giving them gifts and awarding them honorary titles.
In the future you can ensure someone dependable will become your regent by awarding them the "Designated Regent" honorary title. Be sure to choose someone skillful since during their regency their stats will apply to national stats, not yours.