First of all you should use some clay on your paintwork otherwise any bits of tar will just cause more scratches when moving your pad about whether this is by hand or machine, plus you may as well do every panel on your car otherwise only do it where some light swirls or scratches are will only highlight other marks on the panels that you don't do.
The process is wash, dry, clay (if necessary), wash, dry, polish, wax/seal.
I would always polish the whole car, unless you are trying to get rid of a specific new mark/scratch.
Are you hand polishing or machine polishing?
And with regards to clay, I suggest you look in Halfords at the Megs Quick Clay Kit. This gives you a decent clay bar, some detailing fluid for lubrication and a MF towel to buff with. Have you clayed before and do you understand how/why it works?