We are created in God's image. In Genesis, God says "let US create man in OUR image." This says a lot to me since we are created in God's image and he is saying "us" and "our"
This is only one example where God is shown to be plural, but one.
Also, in Genesis, it say "God created the heavens and earth" but in Colossians, it mentioned that all things (not all other things, mind you) were created through Jesus. So if Jesus created the heavens and earth, then he must also be God.
Those 2 things were proof enough for me.
Of course, in nature, an egg proved it to me also. There is a shell, a yolk and the white. All one egg.