You can use the AdWords keyword planner tool to search for keywords and ad group ideas.
Find more information here - https://support.google.com/adwords/answer/2999770?hl=en
Re: Adwords[ Edited ]
June 2016 - last edited June 2016
A few other tips that I have picked up:
1. Always start with the Keyword Planner
2. Try entering the keyword into the "Add your Keyword here" section (just like when you are adding new keywords to an adgroup) after choosing an AdGroup Google gives you some Ideas that they feel are useful to you.
3. Start typing your keywords into a Google search query and see what Google suggestions are as you are typing
4. after searching your keywords on google scroll to the bottom of the page and Google will give you "Searches related to (your query)"