As reported by several sources, there are some people who are claiming that Bill Gates tweeted that Donald Trump is going to be one of the greatest president like Ronald Reagan. There is no evidence that Bill Gates made this tweet (PolitiFact even tried to find it on the Internet Archive). It is believed that someone used a web site to create the fake tweet. The following fake tweet from Donald Trump was created using one such site.
The lesson to be learned here is that it is possible for someone to create a fake tweet or even a fake Facebook post. It is important for you to be skeptical about the veracity of even seemingly authentic tweets and posts on social media. If there is any doubt, it is easy to go the the alleged source and verify whether or not the tweet or post is authentic.