Let $M$ be a smooth manifold and $C^{\infty}\left(M\right)$ be the space of all smooth functions on $M$. My question is
(a) What is the standard smooth structure that makes $C^{\infty}\left(M\right)$ a smooth manifold? That is, how can we define coordinate charts on $C^{\infty}\left(M\right)$?
(b) Let $f\in C^{\infty}\left(M\right)$, what is the tangent space $T_{f}\left(C^{\infty}\left(M\right)\right)$?
Thank you.