Preperation: Removing flash and mold lines can be difficult. A small file and fine sand paper can give trimmed areas a smooth look. See the pictures below.
   Assembling large parts usually requires some filing on the joints to give better surface area connection. Pining is also a good way to keep parts secure.
Primer does not have to be thick. White is better for amateurs. Another smooth coat of any color after the primer is sometimes very beneficial. Citadel spray paints are available in many colors for this. It can make the texture smooth while speeding up some of the painting.

The first step is deciding what colors to use and how you are going to use them. If you're not sure just paint medium colors (not too dark or light) and take it one step at a time.

Washes should be as thick as possible and as close to the undercoat as possible while still getting a wash effect. Inks aren't recommend if you are still learning to use paint. It is now common to avoid washes altogether and use highlights or transparent layers.

Highlights are easy enough. Don't use thick paint. Drybrushing can still be a little wet and this gets a smoother look.

Painting with thin transparent layers of paint can be difficult and has a very different effect than drybrushing. This is the slowest method of painting using the paintbrush tip to highlight every detail. If can give a better fade from one color to another.

Bases: Here are three good ways to get good bases:
Painted: Glue something to the base that has sharp edges or deep crevasses. The red stones shown are a good example, they started as one piece and got hit with a hammer a few times. Paint and wash it with dark colors and highlight it till it looks like it has depth. Sand is good for futuristic bases but doesn't usually look realistic and it's very common.

Hobbyist: at the risk of looking like you're on model railroad terrain you can paint your base and glue on flock and static grass. This is OK but doesn't always look good close up. Small bushes can add some color.
The chefs touch: Use cracked pepper and hard polenta corn meal first and paint it. Then add some unique inexpensive dried herbs to look like dried leaves or underbrush. Experiment to find what suits you.

    Taking Care of your paints is not always easy. If you use wide mouth jars with screw on lids your paint will get very thick over time. Keeping all your paints all at the same thin consistency is important for getting good results. Some painters even use an ingredient that makes all their paint dry more slowly but it seems safer just to get better paint containers. Empty plastic eyedropper bottles are at many art stores.

Metal - There is no perfect way to paint metal. Metallic paints don't shade well and can look glittery close up. Using nonmetallic metal looks good at some angles but looks plain sometimes and is harder to do. NMM is painted somewhat like metal in a picture. Look at the way it is actually gray, blue, and white instead of silver in a painting or illustration. Some painters have even started puting reflections in the NMM. Some have als resorted to polishing the unpainted metal of sword blades and helmets at least on large figures.

Conversions : Confrontation miniatures are good for conversions but lack the rules to equip miniatures with different weapons unless you want to swap artifacts. Pinning uses a metal pin to join parts making them more secure. It isn't needed for all conversions. Conversions can use items you wouldn't expect.
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This unfinished snow base was an experiment using superglue and confectioners sugar.