Answer:
First you analyze all the data that your program needs. You can do this by drawing UML diagrams or whatever you find convenient.
In your case you'll model an item, a stock, supplier and user. You may not end up implementing all of that, but it's good to get a feel for the extent of your assignment.
You can then define use-cases, i.e., tasks or flows through your program, in your case these would simply be the tasks of your 5 menu options. You can write down what happens to the modeled data for each of those flows.
The combination of the use-cases and data modeling will make clear what data structures you'll actually need. For example, the role of the supplier name is unclear. From the requirements it doesn't say if that has to be stored somewhere.
Note that we haven't talked about programming yet at this stage. This is your analysis.
Then you choose a programming language and start implementing. Test corner cases and unexpected input (like empty lists, quantities of zero and negative).
