Start by going to
yandex.com/images/ and using their image search.
Fuck google image search, that shit never works.
If the image is hentai or an anime image, use
iqdb.org.
For pages from a manga or doujin, use
saucenao.com.
If the image is an anime screencap, use trace.moe.
If all of the above doesn't work, then you have permission to ask for sauce.
