first of all you have to do onpage optimization , include your main keywords in title , description. Then try to generate backlinks from same theme sites. if your site is new then generate sitemap and submit sitemap to google webmaster tools.
Do analysis keywords for your web site relevant and make meta tags then put those tags and build back links for your site.
Use https://adwords.google.com/select/KeywordToolExternal to find out your niche related keyword & write article with those. Submit your article to directory sites.
To choose the keyword for your website you first need to do search in Google keyword tool and choose the keywords that are less competitive but could give you the traffic.