I don't know which posts I should give karma to and whether karma has limits to sharing.
Since you are teleported from bitcointalk, it's just merits, sending merits to high quality posts, helpful post and informational is enough for you to say that the post should +karma worthy or merit worthy. The opposite of it is -karma worthy, more like spam and off-topic.
It's not matter of which post, it's on what you read and your own judgement. About the limits, well, there is, 1 karma (positive or negative) per user in 10 hours, either you gave karma in post or its profile. Well, karma descriptions are all over on this board and stickied threads, you should read about it.