If you want to give someone the exact link to a tweet, click on the time that you sent it on the bottom of your tweet (it shows up in Twitter proper on the web). This will lead you to the direct link to your Tweet. You can either right click on the time and "copy link location" or you can click on it and copy the link from your browser bar. Then if you are trying to link to someone or entering a giveaway and want to give the exact tweet, you can paste it into the comment box or whatever you are trying to do.
Here's what the link will look like: http://twitter.com/anniekawaii/status/3145654488