Color help and advice needed for newbie!!

Hi

I hope someone can advise me on which color to use on my hair.  There seems to be a lot of great experience here.

I have been coloring my hair (dark blonde/grey, 7) at home using a shop bought Recital Helsinki Light Ash Blonde (10) for years.  I manage to section off my hair and do the roots without getting color build up on the ends, and without being 'bigheaded', I think I do quite a good even job. The only thing is, the roots come up quite brassy, although the ends are quite a nice natural creamy blonde color. 

I want to go to Sally and buy proper salon tint, but I don't know what to get.  I know I need the liquid white developer, and the smaller bottle of liquid color.  Can someone please advise me which product to go for and maybe even a color suggestion?  I would like to keep to a 10, in a pale beige creamy blonde, no too ash, and no gold,brassy hint.  

I have seen cardboard boxes with 'tubes' of color...but I am used to using the liquid color that comes with the shop bought dye. also need advice on how much to buy, and how much of each to use as I am not sure of the ratio of developer to color.  Basically, I need step by step instructions before I put it on my hair!!

Many many thanks in advance

Lesley