Do a search in this forum for “sticky footer”. You will find quite a few very informative discussions about making the footer stick to the bottom of the page (using the proper structure for the html too, not using tables).
The sticky footer will keep the footer in the viewport when the page has minimal content. Then as content exceeds the viewport height it will push the footer down like a normal footer.
If you are wanting the footer at the bottom of the page at all times then you would need a fixed footer. Use with caution though as it can obstruct other content if that content is not padded out to account for the footer height.
For a position fixed footer just wrap your existing footer in a new div and fix that new div.