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.
You should hire an Engineer who knows how to do this and you should read forum policies here which begin as follows:
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