Hire someone who knows what they're doing or learn it yourself by searching for the relevant resources (i.e. SEO Guide, how to do SEO, SEO tutorials, and plenty more) that can aid you in your quest for superior Search Optimisation capabilities.
First take a look on the website backlinks and if possible then remove the backlinks from Google webmaster. Your website keywords ranking could be hit by Google algorithms. If your website is having bad backlinks then it needs to removed from your website otherwise it will get down day by day.
I would look at your backlinks first. Use: https://ahrefs.com, take a look at your anchor text and all your links. If you can find any spammy links use Google disavow tool as a last resort if you cannot manually remove the links. There is lots of information about SEO. For a beginners guide try: http://moz.com/beginners-guide-to-seo
But as with everything with SEO, trial and error and patience is key.
First analyze your links in google webmaster there is an option internal links go there and check out each link one by one, remove those who are unnatural ,low quality,irrelevant
You can also notice that if a webmaster is showing some message about links then you have been hitted by manual penalty, if there is no message and you have seen sudden drop in traffic then algorithmic penalty must.
Also analyse your analytics account if there is sudden drop in traffic.