I started a new job but I just want to leave without warning and go back to my old one.
Why do you want to quit this job so suddenly?
I like the environment and the people better in my old job
That is very irresponsible of you. You shouldn't start a new job knowing you will quit it soon. That leaves the company a person short and having someone else to make up your work.
I really like my old job though
Most people would frown on this. You know could just negotiate with your boss for better pay right? That would avoid so many more unnecessary steps.