You unlock new buildings as you level up. Once you hit Level 7 you will unlock the ability to build a Pop Star Dorm. Once you have one you can purchase a female Pop Star using coins, but the male costs diamonds. To get the male, send two female Pop Stars to Club VIP for the best chance at the male one coming.
After you do that, you have two additional party options that you can unlock. You can unlock a composer by partying a Pop Star and a Director, and you can get a Fairy Tale by partying a Composer and either a Model or Celebutante. Alternatively, party a Composer with an Action Hero for a good chance at the same thing.
If you are looking for a rare gender of a certain type, such as a female Fairy Tale or a female Action Hero, your best bet is always to take two of the more common version of that type, such as two male Action Heroes, and then send them to Club VIP to party. Sending two of the same type will always have a good chance of cloning that type, and the more you party the two together, the higher the chance of getting the rarer gender.
Need to get more decorations because you don’t have the right ones to complete a quest? Start buying expansions. Oddly enough for this game, buying expansions is the way to unlock more decorations, as opposed to everything else which unlocks when you gain player levels.
To gain experience points quickly, stick to the main quests, because they are the shortest quests and the ones which will get you experience fast, and make sure to keep your main character, Ethan, and Addison leveled up in the workshop because they are the characters most commonly used in the main quests. You will know the main quest because the rewards always include experience points.
Once you unlock the Pop Stars, though, the side quests become more important because even though they might not have the experience points to go with them, they typically have bigger rewards with diamonds and the like at the end of the quest series. However, remember that you do gain five free diamonds every time you gain an experience level.