Find a continuous function $f$ defined on $\mathbb{R}^d$ such that
(1) there is no polynomial $P$ in $d$ variables such that $|f(x)| \leq P(x)$ for all $x \in \mathbb{R}^d$.
(2) The distribution $\phi \mapsto \int \phi f dx$ is tempered.
I just learned tempered distribution, but I don't get the sense of how does a tempered distribution look like. So I don't have any idea of constructing such function.
You will need something that oscillates wildly, yet defines a tempered distribution. The idea is to take something bounded (hence tempered), but with a very capricious derivative - and since derivatives of temepred distribution are tempered, we will obtain what we want.
Consider, for example, $f(x)=\sin(e^x)$. This is a bounded smooth function, hence $f$ is tempered: $f\in S'(\Bbb R)$.
Its derivative belongs to $S'$, too, as a derivative of tempered distribution.
Finally, $f' = e^x \cos(e^x) \in S'(\Bbb R)$, and it can not be bounded by a polynomial for obvious reasons - an exponent grows faster than any polynomial.