Dear Ali,
Well there are various tools which are useful to the study of convex polytopes. The following list is perhaps not complete and it certainly should not be frightening. (I dont know very well various of these tools.)
- Basic tools of linear algebra and convexity.
The notions of sypporting hyperplanes, seperation theorems, Caratheodory, Helly and Radon theorem etc.
- Combinatorics
Some of the study of convex polytopes translates geometric questions to purely combinatorial questions. So familiarity with combinatorial techniques and methods is useful.
- Graph theory
As Joe mentioned the study of polytopes in 3 dimensions is closely related to the study of planar graphs. There are few other connections so it is useful to be familiar with some graph theory.
- Gale duality
The notion of Gale duality is a linear-algebra concept which privides an important techniques in the study of convex polytopes.
- Some basic algebraic topology
Euler's theorem and its higher dimensional analogues is of central imoprtance and this theorem is closely related to algebraic topology. Another example: there is a result that the [d/2]-dimensional skeleton of a simplicial polytope determines the entire combinatorial structure. The proof is based on an elementary topological argument. Borsuk-Ulam theorem has various nice applications.
- Some functional analysis
There is a result by Figiel, Lindenstrauss, and Milman that a centrally symmetric convex polytope in d dimension satisfies $$log f_0(P) \cdot log f_{d-1}(P) \ge \gamma d$$ for some absolute positive constant $\gamma$. The proof is based on a certain variation of Dvoretzky theorem and I am not aware of an alternative approach.
- Some commutative algebra
Several notions and results from commutative algebra plays a role in the study of convex polytopes and related objects. Especially important is the notion of Cohen Macaulay rings and results about these rings.
- Toric varieties
Understanding the topology of certain varieties called "toric varieties" turned out to be quite important for the study of convex polytopes.