Giving rep is something you can only do on the desktop based version of the site - cant do it from a phone or a tablet unless you log into the desktop version of the site rather than the app version.
As for how... You see this?
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At the bottom of a post on the left hand side, above the tab that says 'reply to thread' you'll see a star and a triangle sign. If you click on the star, you'll be able to give rep (the triangle is for reporting dodgy posts - spam, that sort of thing).
And the brown stars - they're actually supposed to be bronze, I believe. They're related to the amount of rep you've received and how many posts you've made. When you've got a lot of rep, they start to turn silver. I think it's also possible for them to turn gold.
Hope that helps