Better Content
This is the most important and most obvious part to start with. Google analyzes your content and tries to show most relevant pages to search phrases first. So the aim should be to have relevant content on your website, including a focus on the most crucial key words for your business and services so your clients will find you when searching for those.
Better Code
Google & Co. are getting more and more strict when it comes to punish coding errors and similar. Also Google starts to put more and more meaning into mobile ready websites.
Besides that there are many ways to improve the code of your website by adding meta information, readable for the bots of search engines.
Other stuff
The algorithm search engines like Google use to determine the ranking for your website changes more and more often. However one thing that is still just as important as in the past is creating backlinks. Backlinks are links on other website that lead to your website. The more websites link to yours, the more important your website appears to Google & Co.
Also it is always helpful to give Google as many information abut your website as possible. Using so called XML Sitemaps with a list of all your web pages, using the Webmaster Tools to improve Googles crawling habbits and even register your business at Google MyBusiness can have an enormous effect on your ranking.