Usually you go up to someone and say "Hey,want to go out with me?" Or "Would you like to have dinner/drinks with me sometime?". "Want to go see a movie together?" There are many ways to ask someone out on a date...the list is a long one, so I just gave a few common examples.

Find something you have in common, or would like to introduce to the prospective date and ask if they would like to attend with you.