Suppose I made a tag and it is used by many people everyday, so will it increase my reputation?
And also, suppose no one uses it even once for a long time, i.e. 6 months, then?
Creating tag does not give you any reputation. However, if it is used at least on 50 questions, then you can earn taxonomist badge.
Another tagging related activity is editing the tag-info (the tag-wiki and the-excerpt). For users below 20k this might bring reputation - assuming your suggested edit gets approved. (And there are also badges tag editor and research assistant.)
Having said that, it is better not to be too hasty about creating tags. If you are not entirely sure whether the tag you want to create is a good one, you might ask on meta. You can ask in the thread which was created for such purposes. However, if you suspect that your proposal will require longer discussion it is better to make a separate question. There is also the tagging chatroom - however not that many users follow this room, so if you ask on meta you have probably much better chance to get some response.
For some advice concerning creation of tags, see also: How can we create a new tag in a Stack Exchange site?
You have also asked what happens if a tag is not used for a long time. If a tag is used in no questions, it is removed during the next 24 hours. If a tag is used only in one question and it is at least 6 months old, it will be removed unless somebody created the tag-wiki for this tag.