The reference implementation of Python, CPython was indeed written in C, but saying that Python is written in C is an oversimplification:
- There are implementations written in other languages, like Jython (written in Java), IronPython (written in C#), PyPy (writen in Python), CLPython (written in Common Lisp), Psyco (also written in C), Stackless Python (written in C and Python) and Unladen Swallow (written in C++)
- Although the CPython interpreter is written in C, it is possible to write modules for it in C++ or Cython (not to be confused with CPython), as well as C
- What language a language's interpreter is written in is only important if you want to write modules / extensions to the interpreter itself, it has nothing to do with the language
Several languages (like Java, PHP, C# and others) are referred to as belonging to the C family, that has nothing to do with what language tools (compilers, interpreters) for said languages are written in but it means that they have very similar syntax to C. Python's syntax is very different from C, not only does it not belong to the C family, its actually quite far from it.
Apart from the CPython interpreter, the only other relation that Python has to C is that they are both multi-purpose, multi-paradigm programming languages.
Whichever one you choose to learn first will greatly help you learn the other one, and that's true for every programming language, as the one you learn first will introduce you to programming concepts and ways of thinking that are common in every language.
Python is generally regarded as a higher level language, whereas C as a lower level language, meaning that Python is closer to what we humans consider friendly and C closer to what the machine considers friendly, so Python is a little bit easier for beginners to start with.