Best way to start on visualizations?

Well there are two areas you need to have a working understanding to be successful. The first is basic chart types - this isn't a Looker thing, you just need to do a little research. Charts map to certain questions. So if you are looking at say churn across a product set, you would want to know the drivers of that and perhaps a stacked column chart might be appropriate. If you want to see gender preference for a product, maybe a pie chart would be good. As to Looker, they have most common visualization modes available. You should not find anything missing. You can play around with different visualization in the Explore area. Lastly, I would advise staying simple in your use of viz and dashboards. This allows you and your viewers to develop an understanding of the insights you are showing and over time you can evolve it.

Looker has a built-in set of visualizations that you can automatically apply to any dataset that they work with. Once you setup your initial connection to a database and build a basic LookML model applying visualizations is very easy. Looker also has advanced options that allow you to apply the D3 suite to data, but it requires a bit more technical expertise than the base included suite.