What was the First Order trying to do in tfa?

Can anyone explain what the government was at the beginning of the film? So the Senate was back, there's a republic, and the First Order were like a rouge state, terrorists? But it was said that the republic supported the resistance? Well how are they a resistance then? The FO blew up all government planets, so what are they wanting to happen? If the central government had no military and were dependant on the resistance for defence, why didn't they destroy the resistance and then easily take over the government again? It seemed really quickly dismissed what was going on and why The FO wanted to destroy the planets...Any explanation would be appreciated, I'm relatively new to this Star Wars stuff. via /r/StarWars http://ift.tt/2w2FhBg