Are these the first models you're painting? If so you have one hell of a steep learning curve to get up before you reach the point where they look like on the box. A few tips to start you off with - never paint out of the pot. Ever. The paint is too thick it will make your minis look bad and it will kill the detail. So, thin your paints. Water does well, a medium (Lahmian Medium or equivalent) does better but if you're just starting water is great. Do yourself a favour and make a wet palette. It's not essential but it does help a lot and you will hey to a point in your painting career when you'll suddenly find yourself needing one and it would help a lot if you learned how to make one or had one about already. Don't worry, wet palettes are super cheap, all you need is a plastic container, some paper towels and some parchment paper and you're ready to go, watch a tutorial on it.
Other than that watch lots and lots of tutorial videos and experiment. You WON'T get the paint right the first time, no one does, it won't look great but keep practicing and you will get there.
Painting is usually - basecoat, shade, highlight (with steps in between etc)
Hexwraiths - get the basecoat on (bone for bone, greeny blue for robes etc). Give the bone a wash of agrax earthshade or nuln oil whichever you prefer. The robes are the tricky part, up to you whether you have them dark first and work up to a light colour or have them light first and work down the shadows. They will require extensive layering/glazing, by which I mean get your paint really thin and paint on the colour transitions. I strongly recommend getting on youtube and watching a dozen tutorials as they help massively.
Lastly - experiment! Try something new! Remember, subtlety is the key to getting a professional look.