How do sims get engaged on the Sims 2?

To get engaged, the relationship status has to be up to 75 [you have to be in love]. Then a box will come up on the screen asking you if you would like to propose. Click yes.

That is for the original Sims. In the Sims 2, you have to love the person first, then you click on them and go Propose>Engagement. to get married, you can either buy a 'Trellisor Wedding Arch' and have both people click on 'Get Married', or you get one person to click on the other and go Propose>Marriage.