in the group of Weil divisors on $C$. More generally if $\mathbb{P}^n$ is compex projective $n$-space it follows the global sections $H^0(\mathbb{P}^n, \mathcal{O}(d))$ is the vector space of homogeneous degree $d$ polynomials in $n+1$ variables. In Proposition II.6.4 in Hartshorne and the examples 6.5.1, 6.5.2 some examples are given.
The book is more than 700 pages long and give all details - again you should check these details if you need this construction. In Example II.8.20 in Hartshorne they write down the following exact sequence defining the tangent bundle of projective space $X:=\mathbb{P}^n_k$: