Acrylic paint is a fine art paint. Unlike oil-based paint, another medium for artists, you don't thin acrylic artist colors with paint thinner. The result is a hideous mess should you attempt to. There are other thinners to employ for acrylic artist paints. Each has different qualities. Evaluate what you're seeking as an end result for your painting or area of the painting, and choose the thinner that will work best.
Water
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Water is the easiest thinner. It's ubiquitous and usually free. You can thin your acrylic paint down to a watercolor thinness by adding a great deal of it to a small scoop of paint. This is good for staining. Small amount of water added to acrylic make it more workable. This means it's not as tacky and thick. Water will keep the paint matte. You don't need to worry about glossy glare coming from the surface of your painting. It does not have body, though. Since water isn't viscous, it's harder to build up thicker surfaces.
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Acrylic Matte Medium
Acrylic matte medium is a fine, thick consistency. It's thicker than water, but not as dense and viscous as the paint. This means you can achieve a nice, milkshake-like paint mixture, which is advantageous for heavier, richer applications on the surface of your painting. The matte qualities, imparted into the medium, mean the heavier texture won't come at the cost of a glossy finish.
Acrylic Gloss Medium
Gloss medium is extremely similar to its matte counterpart. The viscosity is almost identical. That means thicker paint mixtures in comparison to water. The gloss medium, though, imparts a fine sheen to the painting's finished surface. For a portrait, still life or other type of painting where you wish to achieve a look of shiny varnish, like an oil painting, at the end, use gloss medium in all your color mixtures.
Other Additives
Acrylic lends itself to lots of different types of additives, media or other kinds of thinners and media that have special qualities. Heavy gels can be used to blend with paint. They thin the paint in one sense, the acrylic colors will go further. These gels, though, are actually denser than the paint. Some can be scooped into peaks or even sculptural forms with the paint embedded. Other additives in liquid form, like fabric sealers, prepare the acrylic paint for durable wear when applied to a raw fabric like canvas or cotton.