There are many ways to do it, depending on the field type you are using. The easiest is with the Link or Url field type (link is better as url field is lightweight). You can just set the display formatter correctly, and style it in CSS as a button. If it is in a text field, then you have to output the field in anchor tags setting it as the href attribute value. You can do it in a preprocess node template function in your theme's template file where you rewrite or replace the field value to be an anchor or if you are more of a site builder than a coder then you should look at Display Suite module and use the code field option.
Another easy option is to override the node.tpl.php file for your node type (use the proper override in the naming) and in the template file output the link using the url() function. Depending on the order of your fields you might have to then also hide the field if the content variable is rendered before you need the anchor.
My personal preference is to use Display Suite as it is such a versatile module.