Lumber yards can be a scary experience to the beginning woodworker. Often the price lists are not published and if they are, calculating board feet on the fly then multiplying by the listed price is not something that the average Joe can do. If you are not used to thinking in terms of board feet then just getting the right amount of lumber can be difficult. So, you should arrange plan before go to the lumber yard.
Plan out your project ahead of time and visualize the sizes of the pieces you will need. What pieces are visible and need to be in the primary wood? What can you make from secondary or less than perfect primary wood? Begin to plan the process of going from rough lumber to ready to assemble parts. If necessary draw some diagrams of a board and what parts would come from it. Round up your part sizes to the nearest whole number or even add an inch to account for saw kerfs, wood movement, etc. This is your shopping list. It isn’t a cut list but with it you should be able to buy the number of boards needed then break them down into parts prior to milling.