First go to the romantic interaction section. Keep selecting actions like Flirt, Compliment Appearance, Compliment Personality, Amorous Hug, Flirtatious Joke, or from Holding Hands until you get to the Very Alluring stage( which means that you can take your actions to the next level!). Once there, keep going until you see " First Kiss". Then keep going until your in the extremely irresistible stage. Tell your beloved sim to "Take It steady" and BOOM there dating. Once they're extremely irresistible to each other they can get engaged and then at any time get married.
This may not work if the other sim is: In a committed relationship, In a bad mood, Commitment issues, Or just not the flirty person( The last two are traits. Which means it will be tougher to get them to committ so you will have be BFF's with them first). If your good enough friends with a sim in a committed relationship you can make them break up with there other for you! ( Be careful because the person's EX-wife or girlfriend will now hate you). Hope this help!
For best results use this guide on XBOX 360.