Simply begin your tweet with their username (@username). This also works for people you aren't following.
If someone that DOES follow you retweets you and someone sees that tweet on their TL they will RT you.. and they don't have to be following you... Follow me @ALexisU_
Tweet them.
Send them a tweet and ask them nicely.
By clicking on 'reply' or starting a tweet from scratch and starting it with their username (@username).
Yes you can still tweet them and they will still see the tweet in their mentions.
You can't make someone follow you. If you want him to follow you then you should tweet him and ask him to do so.
You can tweet someone even if their account is protected.
This is not possible, try asking the person with a tweet (to send a message directly to someone type @<twittername> before your tweet, however it will still be public, I think). To request to follow someone with a private account, click on follow and a message will be sent to them to either accept or deny your request.
You receive a tweet when someone mentions you in their tweet.
In order to direct message someone they must follow you, and you must follow them. If this is the case, simply type D:theirtwittername and then your message or question??! send it to 40404.
Begin your tweet with their username (@username)
The only way that your tweets will appear on their timeline is if they are following you. If you want someone to see a tweet but they aren't following you then you should 'mention' them in your tweet (@username)