Compression ratio in basic terms, is the total volume of an individual cylinder including volume of combustion chamber, when the piston is at the bottom of it's stroke divided by the total volume when the piston it at the top of its stroke. If the total volume at piston bottom is 1000cc, and the total volume at the top of piston travel is 100cc, then the compression ratio would be 10:1.
To compute this, you would need to know bore and stroke, deck height (if top of piston is above or below deck surface), head gasket thickness, combustion chamber volume, and valve relief volume (or pop-up volume) of the piston. These numbers all get plugged into a formula, and out pops the compression ratio. There are many websites that have online calculators, where you can just plug in the numbers....
Rocker arm ratio is the ratio between the actual lift of the cam lobe and the amount the valve opens. A common ratio for engines is 1.5:1. This means that if the cam lobe has a total lift of .300", the valve will be opened .450".
This is accomplished by having the pivot point of the rocker arm closer to the cam lobe (pushrod) side of rocker and the valve stem side farther away from the pivot point.
One way of changing valve lift without going with a different cam would be to change the rocker ratio. If the ratio were 1.6:1, the same cam would now have a valve lift of .480".
There's a lot more to it than that, but this should get you pointed in the right direction.
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