To start off, you will probably want to update to the most recent version of Sprout Forms on Craft 2. As of this message, it looks like v2.6.4 is the most recent version:
https://github.com/barrelstrength/craft-sprout-forms/releases/tag/v2.6.4
Sprout Forms manages Fields independently of Craft so you will not see any of your Craft Fields when editing a Form. Fields you create for a particular form will only be available to that specific form.
Once you create a Form, you should see a New Field
button below the initial empty Tab. You will need to add a Field to the Tab before you save in order for the Tab to be saved.