newby...highlighting over color

At age 47 I would love to try to color my own hair at home, especially because the gray is coming in and my roots look very ashy very quickly.  My stylist has been giving me highlights and lowlights the last time I had this done was April.So much$$$$$!!! In July I got my nerve up and used a root touch up in a light golden brown and it seemed to blend well. I used it again last week and now its starting to look a bit darker on top cause there are no highlights. BTW I also cut about 4 inches off my hair and it looks really good.....if I could only blend some highlights in on top Id be very happy. Can I just go with a lighter color, like a medium golden blonde to make highlights, or do I have to buy a kit with bleach. How long do i leave it on for so it will match? Also if I ever wanted to color my whole head...to a dark blonde, could I just buy a box of color and go over what i have now or would I have to lighten it first. My natural color is a drab medium mousey brown with ashy grey streaks. I have quite a bit of natural color underneath cause Ive just been doing the highlight lowlight thing...but it doent look bad. Thanks so much i really appreciate your guidance...I have a few friends who do their own hair and they always seem to look great!