You can create a smaller volume of wort and dilute later to get your volume as you mention. But since your pot is plenty big enough to accomodate the full batch, there are advantages to a full volume boil. First, your hop utilization goes down when you have to dilute with large volumes of water, and secondly your beer can be darker than intended by concentrating the sugars. You'll get a better overall product by boiling the entire volume. Now you need to figure out your boiloff rate for your pot. Add 2 or 3 gallons of water ( be accurate whichever you choose) to your pot and boil it for 60 minutes. Now measure what's left. The amount that evaporated is your boiloff rate/60 minutes.
Steep the grain in 155F water for 30 minutes and strain, leaving all the grain husk behind. Add the liquid to your pot along with enough water to hit 5 gallons + the amount of water that boiled off in an hour (often 1-1.5 gallons) . Bring to a near boil, turn off the heat, and add your malt extraxct. Stir thoroughly to dissolve then bring to a boil for 60 minutes. Add hops as directed, then cool to 65F if possible and add your yeast.
And welcome !