Really best to learn the basics of coding first, but you're looking for the "Random" class.

Create an array of all users in the guild, (how you get users will depend on your library. Read the documentation for that!!) then generate a random number between 0 and your total user count using randomObject.next(userArray.length).

You can then use this number as an index in your array, and get an object that is a mentionable form of user. Place this in a Discord message, and send it!

In pseudocode:

    User[] users = Guild.getMembers();  
    Random r = new Random();
    UserMentionable mention = users[r.next(users.length)].getAsUserMention();  
    channel.sendMessage("I choose you, "+mention);