The largest search engines, Google, Bing and Yahoo, each have fairly simple programs to get started. You determine the key words or search phrase as well as the amount you are willing to spend (per click and/or per day). Most of them offer simple programs that require you to fill out a form and put a certain amount of money down as credit; you can also use a credit card to pay as you go. Like anything new, start small with paid SEM and add complexity (and budget) as you learn more and see results over time.
Not all web traffic is welcomed. Some companies offer advertising schemes that, in return for increased web traffic (visitors), pay for screen space on the site. There is also "fake traffic", which is bot traffic generated by a third party. This type of traffic can damage a website's reputation, its visibility on Google, and overall domain authority.