It takes practice. Look into stabilization techniques (rubbing hands, spinning, etc). These make the dream more vivid and keep you in the dream longer. As for controlling (Stuff like "Spawning in" things or whatever), that requires you to fully expect it to work.
You may need to do some more indirect things. For example to spawn a dream character, you may need to expect them to be behind you, or behind a door, around a corner, etc.