citizenship test

I want to move to the uk in 2008 and take the citizenship test. Who do you see about this, and what happens after you pass it?
and do i have to tell immigration thats my reason for visiting

You can't just come over and take the citizenship test.  Have you lived here before?

no i have not how long do you have to be there be fore you can take it. i downloaded that further leave to remain form (FLR O)
do you need that?

I think you need to look into this a bit more. 

Immigration is a very complicated thing and you can't just decide to move to the UK, fill in a form, and then move.  There are countless threads here that explain what sort of circumstances would allow you the privilage (and it is a privilage, not a right!) to move to the UK.  In summary, you must:

1.  Qualify for British citizenship through ancestory (so your father or mother would have to be British)
2.  Be married to a Brit, or planning a marriage to a Brit and then live here for three years

There are relatively few ways to actually get British citizenship other than the two above.  I'd suggest you spend some more time reading about this before you get too excited about your new British life.  It's not as easy as you may think to come and live in Britian, and it's even harder to become a citizen.