To create stock and purchase entry form in c#

I have to assume ythat you are a complete beginner, so:

In VS, right click your project.
Select "Add...Windows Form"
Give it a sensible name "UserStockEntry" or similar.
Press OK.
You can now fill the form with the appropriate labels, text boxes, comboboxes, etc. to let the user enter the data.
I would also suggest that either each user entry should have an associated Property, or the form should have a StockEntry class variable (if your code is set up that way) to return the data to the calling form. I would also suggest that the Setters on the properties should load the textboxes etc., so the same form can be re-used as a stock edit form as well.

To display it:

UserStockEntry form = new UserStockEntry();
form.Item = "A widget";
...
if (form.ShowDialog == DialogResult.OK)
   {
   // Save the data away.
   string item = form.Item;
   ...
   }