What camera are you using?
As K_Duffer said, if you adjust shutter speed while your camera is in an auto-exposure mode, it's going to try to compensate to get the same exposure as before. Shutter speeds as slow as 1/8th or 1/4th sec are probably too slow for hand holding if you want sharp, in focus photographs.
To increase exposure while in a manual exposure mode you can increase aperture size, slow down the shutter, or increase ISO, or a combination of any of these. Take a look at the sticky post at the top of this forum, and get a basic understanding of how aperture, shutter, and ISO are related to exposure.
It's also important to understand how your meter works, and if you have a choice of metering types (matrix, center weighted, averaging, spot, etc...).