This is how my equipment is setup, yours will likely differ.
Under "View", select "Equipment"
Double click on "My Equipment".

My mash tun is sold as a Willow 55L esky. It actually will only hold 48L. But i digress....
The 2kg weight is a rough guesstimate, weigh your mash tun if you want a more accurate measurement.
Select the material it is made out of - in my case plastic.
Now to the more user specific settings.
Lauter tun deadspace - easily determined by filling up your tun with water and then draining until the flow stops. Measure the volume remaining. Beersmith will then add this to your sparge water
Top up water to kettle i have set to 0. Why? Have always hit pre boil volume so i dont need a top up.....
To the boiling part - check the box - 'calculate boil volume automatically'
The evaporation rate is user specific. Easiest way to check prior to your 1st AG is fill the tun with 20L, boil for 60 mins. Measure remaining water and calculate boil off rate (Rate = volume evaporated/initial volume x 100). Note - this depends on how vigorous your boil is so make a note of your regulator setting. I have had to adjust this a few times so dont be too worried if you are out by a few litres the first few batches. (I boil for 90 mins - you dont have to so to start with do 60 min boils. 60 vs 90 min boil are a debate we dont need here!)
Loss to trub and chiller - this is where it becomes a personal choice. I set this to 0 as i know beersmith has an error in the software and so doesnt take this value into account when calculating IBU etc. IMO, set this to 0 and whatever volume you have left behind after draining the kettle, add this to your total desired volume. This way you hit your target volume with the correct IBU/OG etc. You can use the volume left behind for starters as some people do. My loss is only ~1L as i tip my kettle during the drain!
The cooling loss i have set to 4% as i lose ~1L in a single batch.
So for a 21L batch (in which i aim to get 20L into the fermenter) i need a pre-boil volume of ~27L.
Once you have all that set-up, you should have a good chance on hitting your volumes. Efficiency will determine your OG and this is a guess on the first AG. 70% is a good start, i started there and thats what i get every time. Whilst it would be nice to be higher, consistency is far more important in making sure you hit your target OG.
Hope that helps out those new to AG and beersmith.
Cheers
DrSmurto