Angel, setting up the table to store the stocks is easy. I would just have:

StockName - text like "MSFT"
CustomerID - number, foreign key

That's really it. Then you can create a subform inside your Customer form to enter in what stocks they have. In fact, I recorded a video on this exact subject a week or so ago:

Form Subform Tip

Now making a query to show each stock and who owns it is easy too. Make a query with your Stock table and your Customer table (related by CustomerID) and then set a criteria for the stock name. Again, here's a tutorial:

Parameter Queries

I cover all of this stuff in my full Access courses. I see you've taken only Access 101. There's a LOT of good stuff in 102 through 201 that would answer these questions and more.