If you haven’t used acrylics before then you need to experiment a bit.  I have settled on a system of pre shading.  I undercoat white (using a Halfords white aerosol primer) thena wash using GW Agrax Earthshade or Windsor & Newton Peat Brown ink.  I thin the acrylic paint to let the undercoat and shade show through and block paint. 

Others undercoat black and paint the high spots in a denser colour.  Vallejo is good for that as they have good pigment density.  You do need to give them a really good shake and I second the suggestion for a ball bearing in each bottle.  I have gone from old Humbrol Enamels to acrylics like you have.  I wouldn’t go back if you paid me.  The colours mix well and cover well.  I do suggest buying a drying retarder as acrylics dry faster than enamels although water works almost as well and you do get used to the shorter set edge time.

I use a grey primer, then apply wash and pick out the details that now stand out with colour - you can see some examples by searching for nerad games and looking at images. This technique works perfectly for me using Baccus figures due to nice detail.

Since you first posted I have changed my technique, having been put onto this by a hobby shop owner. Rather than dilute with water I now use Vallejo acrylic thinner. This thins the paint, making it easier to apply, without dispersing the colour, which water does apparently.

FWIW for my paining over the past few years, 2mm - 35mm I have used inks from liquitex and daler rowney.  I don't claim to be anything other than a wargame level camoflager, but they work.

So far I've only got a spray can of Vallejo black primer and one or two bottles of paint, and the rest of my paints are still Citadel. I will switch to Vallejo and possibly Army Paint in the future. The primer is great, anyway!

One of my wargaming friends is also an industrial chemist and his mantra (which I follow) is don't undercoat until you have used a primer, as primers are made to adhere to metal better than ordinary paints. Once primed, and I always prime using Vallejo white primer, then undercoat whatever colour you like - but not too thick.