It is a little complicated at first. It is usually best to start by looking at a few. Basically you start with the square footage at 3va per sq/ft, apply a correction factor to that and then start adding in the fixed equipment.
There are actually 2 methods you can use.

There are examples in the back of the NEC book but you can also find some work sheets and even a couple Excel templates on the net.

Here are a few examples.

https://www.modestogov.com/forms/pdf/bds_res_electrical-load.pdf

http://www.zenfixit.com/load_calculations.shtml

https://www.mikeholt.com/documents/calculations/formulas/ResidentialLoadCalculations.xls


Greg Fretwell