How do you get married and Then go to the next level with somebody and..

Right first of all how to get married: you have to build up a loving relationship with two Sims (the actions like Dance, be romantic) until you get to the point where you are partners, then you have the option to get engaged, tap that and a list of rings will come up, choose an expensive one because that will give you a better chance for them to agree to getting married. Once you are engaged, build up your relationship and then you can get married.
Second of all how to get to the next level with a sim: if you want a friend relationship tap on the green icons like, be nice and be funny, if you want a loving relationship tap on the icons like, be romantic and Dance, if you want a nasty relationship tap on the icons like, be rude and complain
Thirdly how you become pregnant: in this game Sims don't become pregnant but a married couple can have a baby, to do that, buy a cot from the toy store on the map, and go home and go into your inventory and place the cot, you can now tap the cot to create a baby. The baby takes a day to arrive and then tap the cot again when it is finished and the same way you created a sim, you create the baby. If you have a stove you can bake a birthday cake and then tap the baby and tap have birthday, the baby will then grow into a toddler, and then you can bake another birthday cake and tap have birthday on the toddler and it will grow into a preteen. They cannot grow into adults, but that might come in a further update.
Lastly how to buy a pool: if you tap on the store whilst you are at home, tap on the outdoor section, and tap on the pool tool, you can now build the pool as big or as small as you want!

Hope this all helped! If you have any further questions I will try to answer them!