Find out what terms your potential customers use to describe the products you're selling. You can do this by participating in forums where those customers hang out, by sending out email surveys, by reading trade publications or other magazines/newspapers/newsletters they read, etc.
You can also visit the sites of your competitors and see what terms they seem to be using in their on-page content. (But that that with a grain of salt. Your competitors may not have done sufficient research to know the best keywords themselves.)
You can also use Google's own keyword planner tool
to brainstorm new keywords. If you don't already have an AdWords account, you'll have to sign up for one, but after that you can use the tool for free without actually having to run any ad campaign.
There are also many other online keyword research tools available (Wordtracker, SpyFu, SEMrush, etc.). The better ones will cost some money, but most have short subscriptions available so you don't get locked in to spending too much. Just keep in mind that not one of them has access to actual Google information, so you have to use your own discretion when evaluating their suggestions.