。。
Me too. How about tennis?
Literal
I-[also-も] [is-だ]-[insistence-よ]. Tennis [topic-は] how?
A two-part casual response. 僕もだよ is elliptical — the implied predicate is whatever the previous speaker said. Then a suggestion with テニスはどう — どう is the question word 'how,' often contracted from どうですか in casual speech. テニス is an English loanword.