How can I get mods from the Steam workshop?
In your Steam library, select Civilization V and you will see a button labelled "Browse the Workshop";
Clicking this button will take you to the Steam Workshop, which looks and feels the same as the Steam store in that you can browse through items and download the ones that you want;
Once you have found something in the workshop that you wish to download, click on the 'Subscribe' button (big green button similar to the 'Add to Cart' button). This will add the content to your subscriptions list. Some games may require you to restart the game to download subscribed content.