You can measure the opening for a storm door and save yourself some aggravation. The most common standard sizes are 36"wide by 80-81"high for a front door, and 32"wide by 80-81"high for rear or side doors.
The exterior trim around entry doors is referred to as brick molding and all measurements should be taken from the inside edges of that trim. I've found that older homes invariably have headers (the top of the frame) and thresholds that have settled a bit and won't be level. A small variation (1/4" or less) usually doesn't cause a problem as minor installation adjustments can be made.
The newest entry doors seem to be coming in a bit shorter and I've found several that won't accept a standard off the rack storm door. Just under 80" is the absolute minimum and if the threshold isn't level it's really tough to make a standard door fit.
I recommend taking time to have a cup of coffee, or other favorite beverage( in moderation of course) and thoroughly understand the instructions. Take your time, check your measurements. You can do it if you're patient.