Analysis your keywords, look for some cheap keywords that doesn't have high competition, setup a campaign and keep followingup how it performs and change as needed.
Google Ad-word is a great tool for keyword research, you can analysis keyword and find low competitive keyword which will rank on Google fastly.
^^^ useless comments. 1. Depends on your target market. Is it a local, regional, worldwide market? Utilize the GEO targeting feature. You can choose which country, state or even down to a specific city. For example if your business is local to a city, there's no reason to show your ad's to the world. Showing ad's beyond your market will waste money on useless clicks. Target ad's for your market! This feature gets overlooked a lot. 2. Another feature that gets overlooked and can be costly. Use the "Negative keywords" option. Let's say for example you have a residential plumbing business. You would want to put words like these in the negative keywords area, Plumbing school, RV plumbing, Plumbing jobs, Commercial plumbing, plumbing tools, etc. Because your not advertising a school, work on RV's, hiring, doing commercial work or selling tools. Using this feature will help save money by not having un-relevant ad's showing and being clicked on. 3. Choose specific keywords/phrases and stay away from short broad names. Example again, plumbing business would benefit more from a keyword like this "plumbing repair Atlanta" than "plumbing Atlanta". The first name tells you they're already looking to call someone to do a repair as with the second one, it could mean anything. Like looking for companies, schools, tools, info, etc. Get specific with keywords! You want conversions not just random clicks. I didn't mention much about keyword research because that's just "common sense". That's like saying how do I get online? Turn on your computer first.
Doing keyword research, using free tools like SEMRush & KeywordSpy to get a sense of what competitors are doing/prices they are bidding.