Quick answer
Я шути́л.
ya shutíl — шути́л for a man, шути́ла for a woman — and шучу́ for right now
The options
Я шучу́.I’m joking. — right now
Я шути́л.I was joking. — male speaker
Я шути́ла.I was joking. — female speaker
Э́то была́ шу́тка.That was a joke.
Без оби́д.No offence.
Present or past?
Я шучу́ corrects the misunderstanding as it happens — “no no, I’m joking”. Я шути́л looks back at something already said. Both are common, and the present one is quicker to reach for.
Smoothing it over
Без оби́д — no offence — and Я не хоте́л оби́деть — I didn’t mean to offend — both do the repair work. Russians also use Я по-до́брому, literally “I meant it kindly”.
What you hear back
Да я по́нял.I got it.
Ну и шу́тки у тебя́.Some sense of humour you have.
Всё в поря́дке.It’s fine.
Typical mistakes
Я шути́л — said by a woman
Я шути́ла.
The past agrees with gender.
Э́то был шу́тка.
Э́то была́ шу́тка.
шу́тка is feminine.
Check yourself
A woman says “I was joking”:
Feminine past: шути́ла.