you can spend a lot of time on core keywords ranking, but high traffic lower turn over rate, you also can choose a long tail keyword but can continue bring the potential customers. which one better?
Let me tell you my experience for my test website: Uniware cheap discount high quality housewares deals products.( i am crazy) when i did housewares this keyword, its' ranking go up fast, until stop on the second page. i found i already spend 6 weeks on this keywords, although Uniware saw the keyword going up, but no extra orders come out, they pay the monthly fee, So, in the other hand, they did not saw the result. and I feel bad when i talking about SEO, they pay me money, but where is the orders? i have to tell them, SEO is useful like investment, but may be their deadline is 8 weeks, and never trust SEO, even Google ranking.
then, i changed my way, long tail easier then core keywords, why not optimize long tail first and get some orders then focus on the core keywords? and the result is also great, some big order make Uniware so happy, there is one more thing need to say, not every company fit for SEO, ranking, what i do is just great for E-commerce, is a long way optimization.
For the services company, the result could be Email or phone quotation.
Try the long tail first, this is my suggestion for you, beginner, just like me try cheap housewares, discount housewares first!