After painting your miniatures, the next best thing you can do is to add something to the base. A well painted mini is great, but an excellent looking base can make a model fantastic and add dynamic scenery or contrasting colors to the paint scheme. Basing is another step and more work at the end of an already long process, but it is well worth your time.
Choosing what type of base to do can be fun, and there's only a few guiding principles. When you first start out try to keep it simple with just one element on the base. For instance, a base covered in sand (as I'll show later in the post) or in grass or maybe rocks. Having sand and rocks, or rocks and rubble/debris, or a mix of desert grass and sand also looks great, and I would recommend trying those more advanced bases eventually, but those bases require another level of expertise.
Also, try to choose a base color that goes with your minis paint scheme. For Genesis Chapter, the brighter colored sand helps to lighten up an overall dark model. Generally models with lighter schemes have dark bases, and vice versa.
Here is a step-by-step process of how I based my marines.
Tools of the Trade. Citadel sand, tooth picks and Citadel PVA glue.
First, I take a finished Marine and clean his base. You can use water if you want. I normally take a clean "dust brush" so to speak that I use to brush away any dust that has settled while constructing the mini. This will create a nice, clean surface for the glue.
The whole process only takes a few minutes, and the end result looks good. It is worth your time to base the models. I am hoping with my next armies to branch out with more creative basing styles.