How do you get Decision Height? I'm looking at an IFR plate and I can't find it, is there some kind of general knowledge? (like always 1,000 MSL?)
1000msl will have you flying underground at many airports

( I'm sure you mean 100agl ;) )
ANYway.. decision height (DH), and minimum descent altitude (MDA), are variables that vary by approach type, and airport's minimums.
Post the plate of your choice, and we'll walk through it.