I've been dying my own hair for about 12 years. I have trouble with highlighting though and leave that to the professionals since it never turns out even remotely like I'm hoping for.
As far as all-over dye, it's very easy as long as you stay within a few shades. If you aren't sure, then I suggest trying a semi-permanent dye like Clairol's Natural Instincts. It washes out in approx 28 shampoos (so in roughly a month if you wash your hair daily) and comes with a great conditioner. It's about $8-10 p[er box depending where you buy it.
I've used L'Oreal Feria dye and love the color and the conditioner. It does seem to give more sparkle to hair so that it doesn't seem like the color is as flat. I've used Clairol Herbal Essences which doesn't smell bad like the other dyes, and that's a plus. I've heard that Revlon Colorsilk covers gray well, and it's also very affordable! I like Clairol Hydrience, too. THe important thing to remember regardless of the brand you use is to follow the directions (especially regarding how long to leave it in) and to use the conditioner! You should also do a strand test too since you haven't colored your own hair before and will probably be a bit nervous about the outcome. The strand test will show you what your hair will look like after you dye it.
Going darker is generally easier than going lighter, though you should stay within a few shades unless you want to risk a dramatic change. (I've sported everything from platinum blonde to blood red to light brown and I'm a natural dirty/sandy dark blonde.)