Every time I start building a house on minecraft it gets dark?

Blocking light from outside will make the inside dark. Including a lot of windows or other openings will help, but only during the day, still gonig to be a problem at night. The way to handle this is to include light sources inside the building, like torches.

Edit: Also, you might not want to go around posting that you are 11, as you are supposed to be 13 or over to be on this site.

Build it during day time

Try to get it built during day time and make torches (a stick with a coal on top) and glass blocks or panes for windows to keep the light in