A discrete random variable has pdf $\displaystyle p(y)=bk^y, y=0,1,2,...$

Find Y's mgf $\displaystyle m(t)$ generally.

I get a bit stuck on what to do..

I know that $\displaystyle m(t)=E(e^{ty})$

so $\displaystyle m(t)=\sum^{\inf}_{y=0}e^{ty}\cdot p(y)$which is equal to

$\displaystyle m(t)=\sum^{\inf}_{y=0}e^{ty}\cdot bk^y$ but I'm not too sure how to expand this out...

thanks for any help!