This is a huuuge question. I think that if you have an engineering background and a decent knowledge of Thermodynamics and simple heat transfer (not the formal course, but the basics as learned in a any Physics course) then you can use HAP, eQuest or similar to estimate it and get a decent interpretation of the results. If you don't have formal engineering training, you probably should not be doing this work, unless is a really small system. Hint: cooling load is much more difficult to estimate than the heating load, if you can't figure out why on your own, chances are you are way in over your head.