The simple answer is that you need to calculate 100% efficiency and then measure your wort and see what percentage of that you got.
You'll need
a chart like this one that gives the potential for various grains.
From the chart; German Pils has a potential of 1.037 (also can be expressed as 37 pts/lb/gallon...that's points per pound per gallon). So if you mashed 1# of German Pils malt and yielded 1 gallon of final wort...the gravity would be 1.037
@ 100% efficiency. But you won't get 1.037...you'll get less...say you got 1.034. 34/37=92% so that's 92% efficiency.
If you mashed 1# and got 2 gallons of wort then you'd divide by 2...so in that case 100% efficiency would be 37/2=18.5 and your gravity
@ 100% eff would be 1.0185. But you'd only get some fraction of that (your efficiency).
Ya know what...better to just read BobbyM's primer.