If $f\in L^p(a,b)$ with $p>1$, then using Holder's inequality with $f$ and $1$ we get $$ \int_a^b|f(x)|\;dx\leq\Big(\int_a^b|f(x)|^p\;dx\Big)^{\frac{1}{p}}\Big(\int_a^b\;dx\Big)^{\frac{1}{q}}=(b-a)^{\frac{1}{q}}||f||_p $$ Therefore $f\in L^1(a,b)$, and in fact $$ ||f||_1\leq (b-a)^{\frac{1}{q}}||f||_p$$