In order to marry somebody, I recommend to get your relation ship with that person at 100. Make sure you love them, and they love you.
NOTE :Your only able to marry sims that live of the free streets. So you can make your own wife if you prefer.
Then, you will need to invite them over. Cook dinner for the person, and afterwards let them go to the bathroom. After they have gone and done their business, watch some televison with them. Talk to them for a little, mabye do some flirting just to be sure. Finally, here it is, you propose to them. This is a method I have used everytime, and its worked everytime. If not, then you have to be doing a step wrong.