[trinity gl 1970]

ok guys i am new to the german look and want to fit big wheels on my car with stock fenders i have to admit all this talk about offset is confusing is there a website that clarifies how to measure offset and if there is can someone direct me there i am not sure when it comes to wheels but i want to run 17x7 on my car with stock fenders and want to know what offset i will need to make them work