Spend a lot of time with some one. Look at their interests and keep asking them out places, they will eventually ask you on a date and you can choose 'i like you as a friend' or 'you bet'. A red heart will then be next to their picture and you have the option of asking them for a candle lit meal or romantic movie as well as the other activities.
You can tell how strong your friendship is by the beam of light connecting you and their picture. To see how long you have been dating double click on the heart that comes up next to their face you can also end the relationship.
Be careful if you don't spend much time with them they will dump you.