So I need to go to wallet in stemgeeks and then stake

yes, or you can use Hive-engine.com, it has a stake button as well.

And I see posts from several others tags are showing up on stemgeeks how do I find out which one is with your tags ?

Anything on stemgeeks.net is indexed and rewarded with STEM and are safe to vote on.

Anything anywhere else, you will need to see if it uses one of our tags:

• stemgeeks
• stem
• science
• technology
• engineering
• math

All these tags will bring the post into STEMGeeks and make it eligible for rewards. That being said, https://stemgeeks.net is the authoritative answer to what is in our tribe as off-topic content (like talking about Bitcoin price but tagging it as "technology) may get muted.