Here's where you start:
There is pay, which is taxable. Base pay is based on rank/paygrade and years of service. Most new 0-1's (ensigns and 2LTs) start out as 0-1, under 2. Prior enlisted might be at a higher rate due to years of service. There are other pays down the road: sea pay, flight pay, combat pay, various bonuses for certain communities, and so on. The sub bonus can be sizable.
Then there are allowances, not taxed. Basic Allowance for housing (BAH), is one of the bigger ones. There is a difference between with and without dependents rate.
Quite often military compensation estimates will include values for the free medical, dental, vision and prescription care provided, with no co-pays, though that is not part of salary. Ask adults you know how much they pay each month for various parts of their healthcare, and you can estimate a value there.
Commissary (grocery store) and Exchange (department store) benefits are often added in with estimated value. They are tax-free, with generally discounted merchandise. For example, I just bought Mucinex yesterday at the Commissary, $11.63, no sales tax. Price at RiteAid, same product, $18.99. Gallon of milk, $2.50, Safeway, last time I was in there a few weeks ago, about $4. Somehow a number theoretically representing potential savings from these benefits is arrived at.
For 0-1's graduating in 2017, regardless of source, AD base pay is listed in the 1 Jan 2017 pay table.
Then, pick a zip code where you might be stationed, such as San Diego. Assume you have no dependents, look up your O-1 BAH. Not taxed, remember, so you gross the whole amount.
Here's your calculator:
Using my old duty station zip of 92118 (Coronado, CA), and 0-1, no dependents, it's $2217/month.
Look up BAS, same approach. That's Basic Allowance for Subsistence.
If you go right to a ship, research sea duty pay.
This is just a quick and dirty back of the envelope description. All are paid the same, regardless of commissioning source, depending on various individual factors. There is a difference between "base pay/salary" and "total compensation."
An unofficial source: