One important thing to remember is the final size you want your finished artwork to be. So choose a suitable working resolution. I usually work in about 10113x5689 (300pixels per inch) as you never know how you want to use your final piece. This will print out fine as A1 or even A0 size. It’s always easier to reduce an image, but enlarge a digital image too much from the original while printing, will end in a pixelated print :(
In my opinion, the fundamental skills to learn as an artist is perspective, lighting and shading... almost anything can look good with great lighting and shading ;)
I normally do most of my painting on a single layer, using a single brush in Photoshop, just the soft round brush, but vary the size and pressure using a Wacom.
There is no ‘right’ or ‘wrong’ way to go about it. Have a look at other artists techniques and adopt one or adapt to suit yourself, it all comes with practice.
Keep that in mind as you often don’t need to go too overboard on details that you won’t see. Your mind can imply detail from feedback it gets through your vision. It’s great you can zoom up in Photoshop, but you don’t need to be photo-realistic at 900% zoom :p This eye looks pretty average zoomed up, you can see rough edges, etc. But zoom out to 100% and it looks fine and takes less time to paint, Boom! :D