Red is not good.
To give rep you click on the little square with the blue line at the bottom left of the post, under the name and avatar. Click approve or don't approve (red rep) and leave a comment. The button with the red line is to report a post to the admin or moderator a post you think is inappropriate, not to red rep someone.
Not sure, but I've heard that the rep you give is equal to 1% of the rep you have. Don't know if that's true, but there there is some correlation between the amount you have and the amount you give. You don't decide how much to give, it just happens.
Hope this helps.