Firstly, you undertake keyword research to ensure you have the kinds of keywords that people are searching for on Google and that those keywords are relative to your business. You can then set up a marketing campaign targeted at people using those search terms. Some of the keywords are highly competitive, so expect to pay a different rate per keyword. You can specify how much you want to pay for which keywords, but a lower bid will result in a lower ad position. You specify a total budget and once your ad goes live it will be amongst the ads at the top of the Google page for the search term being used. You only pay once someone clicks on your ad.