Paint comes in a variety of finishes, ranging from matt or flat, which reflects little light, to high gloss. You can't convert glossy paint to flat without completely repainting with a matt finish paint, but fortunately, the same isn't true in the other direction. You can make any of your matt-painted surfaces shiny without altering the colour of the original paint, other than the perceived colour differences that come from the reflected light. If you haven't already painted the surfaces with flat paint, do this before making it glossy.
Protect any surfaces you don't want glossy with masking tape and dust sheets.
Stir a can of clear gloss medium to ensure the gloss agents and binder are evenly mixed. Gloss mediums are available in the paint section of most home improvement stores and in paint stores often near the wood stains. Use acrylic, not oil-based, glosses to ensure compatibility with household latex paint.
Apply the gloss paint to any areas a roll pad can't reach by using a smooth, thin layer foam brush rather than a bristled paintbrush. The foam more closely mimics the roller's texture, and because gloss is reflective, differences in texture show up clearly.
Pour the gloss medium into a paint tray and apply it to the larger surfaces using a paint roller with a short-napped pad. Longer naps will add texture, which will show when the gloss is exposed to some lighting angles.
Coat the entire surface, and then inspect it carefully by shining a flashlight along the surface so the light spills across it, rather than aiming straight at it. Touch up any spots that you notice aren't glossy under this light.
Use this top coating method rather than trying to mix the gloss medium with the flat paint before you apply it. Mixing will thin the paint, turning it into a glaze and reducing its ability to cover a surface without under layers showing through.