I am considering of prototyping a web based point-of-sale system.
I don't have programming skills but I'm thinking of using this project in order to learn.
I would like to ask you the following two questions:

Do you think the above task is achievable within the period of 6 months (for building a rough prototype of the basic functions of a POS)?
If yes, which programming language would you recommend me and why? (I was thinking of Python)

Your advice is greatly appreciated!
Python is a very quick and productive language to develop in, so that would be a good choice, IMO. Personally I find it the most pleasant language to develop in.
But I think a POS system is a terrible first programming project. A proper POS system covers too many aspects like security, authentication, data storage, client-server. Each of those has its own gotcha's and significant learning curve.
If you want to go through with it nonetheless, chop the project up into manageable pieces that can be built and tested separately. You could start by writing a simple program that accepts text commands from the console and stores the transactions in e.g. a text file or in a pickled Python dictionary. This would be the start of the server. Later you can add a web or GUI front-end, or have the server store transactions in a database.
Creating a web based point of sale system