I apologise for joking around. I'll be serious now.
I'm sorry for being so foolish. I'll be more serious from now on.
Please ignore my jokes. I'll be more serious from now on.
「ふざけてすみません、真面目にやります」= I apologise for joking around. I'll be serious now. / I'm sorry for being so foolish. I'll be more serious from now on. / Please ignore my jokes. I'll be more serious from now on.
ボキャブラリー
apologise, I'm sorry = 謝る、ごめんなさい、すみません
joking around, being foolish = ふざける
serious = 真面目
now, from now on = これから
ignore = 無視する、気にしない
joke = 冗談