BeerSmith relies on you having some measurements figured out for your system. However, since you're just starting out and using extract, a lot of the fine tuning available in BeerSmith won't really apply just yet.
For your equipment profile, just use the 5 gallon brewpot that's available from the list.
Set the batch volume as 5 gallons.
Set the top up water as 2.7 gallons
Set the Loss to Trub as zero.
Set the boiloff rate to 1 gallon per hour and boil time to 60 minutes. This is just a starting point that you'll modify after the brew day to match your actual results.
Set the recipe to "extract" (top center drop down menu in the recipe view).
Enter the ingredients.
The main thing is that your first couple of batches are really about the brewing process, not the accuracy of the recipe. You need a baseline of what's "normal" for your equipment and process and that comes from some experience.
It really isn't all that hard, but the explanation can seem very complicated. Basically, it's like trying to talk someone through all the little steps and possibilities involved in making a right turn. It's a lot easier done than said.