niche means the area which youre targeting, e.g. Make money online, or Weight loss or Celebrity gossip etc. you get the idea. After you pick the niche you have to find specific keywords you want to target, the phrases users type into google search, for this you can use Google keyword tool. When you know you keywords you can optimize you website for them(onpage seo) and start doing linkbuilding(off page seo).
Original content are articles originally written by you or for you by someone else, not copied from anywhere. You need original content, or google won't like your website.