As they said.
I'm pretty new to this, a month into it, so I feel your pain.
First thing is: precision, precision, precision. If you don't paint clear lines, everything else won't save it.
Don't try to do fancy highlighting stuff yet if you don't feel comfortable.
Thin your paints, apply basecoat.
Take a step back and think about your color scheme.. Look at the official ones, they usually give a good overall result; the yellow is too bright in contrast. Ushabti Bone does a better job here.
Use just a little white dot in the outer corner of the eye lenses.
Wash the areas with the appropriate shade as you did with the metal areas. For bonecolours, Agrax Earthshade is the one to go.
After you've done your basecoat neatly and the washes all done, highlight. This is a rather tricky step that took/takes me quite some learning, so don't rush!
For Dark Angels, I use the full range for now.
That is Caliban Green, Biel-Tan Shade.
Follow up with edge highlights of Warpstone Glow, with a final highlight of Moot Green. Then glaze it with Waywatcher Green. But take these steps slowly, it can be quite frustrating, but it will come to you
So far your first marines are way better than my first ones, so keep going!
My last finished one is currently my "masterpiece", have a look:
http://www.dakkadakka.com/dakkaforum/posts/list/557109.page
Im rather confident that, if you keep doing this, you can achieve this result in half the time that I did!