If I applied for a Schengen visa from "country A" and I got the visa but I had some commitments in "country B" and I have to finish it before going to Europe should I get back to "country A" to travel to Europe or can I use the visa from any where?
You can enter the Schengen area from anywhere you want. A regular Schengen visa is not tied to a border point, a specific journey or limited to travel directly from the country you reside in. You can also in principle enter the Schengen area through another member state than the one that delivered the visa. Finally, you can also leave the Schengen area to go somewhere else than your country of residence or your country of origin. The only thing that really matters is that you must avoid suggesting that you might in fact want to immigrate and stay somewhere in the Schengen area illegally after the expiration of your visa.
No matter what you do (i.e. even if you never changed plans and are going directly from your country of origin to the Schengen state that issued the visa), the border police can always deny entry if they are not satisfied that you really plan to leave the Schengen area and they might therefore enquire about the purpose of the stay, ask where you will be staying, want to see your return ticket (since the one you presented for your visa application has presumably been canceled), etc. but I don't think coming from another country than the one in which you applied for the visa would be a big issue per se.
A summary of the relevant regulation is available on the official EU website.