There are a couple of ways to do this. With so little information however, it is tough to give exact information. You can do a DLookup() based on the Primary Key of the Products table to put the information in the Sales table or you can use a query that pulls the Foreign Key from the Sales table and the product description, price, etc. from the Products table. Both can be the right way for certain circumstances, but not in all.
I have actually just recently created a database similar to what I think you need, but again, there isn't a lot of information to go on. A few things that would be helpful would be the Primary key fields and their data types from the products table and the sales table. Any relationships between the tables and what you are using as the foreign key. Also, how you are planning on selecting the product to sell (ie. typing in a product number, selecting a name from combo box, etc.)?
**Edit**
Be sure to read the link that NeoPa posted. It will be extremely helpful as you design your database and save you many headaches.