Self Employment tax help?

OK a little help please as im sooooooooo confused. next week i am going to be starting a courier job that is self employed. It is 5 days a week but for 2 evenings a week i am going to be doing a night job where my employer will pay my tax. so basically i'll have a fulltime self employed job and a part time job where my tax is paid for me. I need help as ive heard you have to register to be self employed and i dont know how to do this and also how it will affect the tax i pay to my evening job. The catch is, in around 4-6 weeks it looks like i could have found a job that is full time and better paid than both the above 2 jobs together and ISNT self employed. My questions are: 1) how do i register self employed 2) how will having a fulltime self employed job affect my 2nd job 3) (most importantly) in 4-6 weeks if i start the new job and stop doing both the other jobs (self employed and evening), do i have to tell my new employer i was self employed and do i have to de-register as self employed? and finally 4) if i am only going to be self employed for 4-8 weeks (max) when would i pay my self employment tax? i know theres alot of details and questions but i dont have a clue and ANY advice/help would be much appreciated thanks.