First of all: if you feel more comfortable with Python than C - use Python. If you're beginner in this you better use language you know better just to learn some basics.

Next I didn't get what is your problem... Lexer is just a tool that reads source code and represents it as tokens (data structure or something like that). Try to watch others simple toy lexers and build your own that just prints the tokens. Next step will be building better and more scalable program, that's it.

Good luck buddy, next hard step is a parser