how to get back to fel base from stormwind?
Once you’ve left the starting area, you can’t go back. (The same thing applies to the Goblin, Worgen, Pandaran, and Death Knight starting areas.) The Legion Demon Hunter order hall is in the same place, but you’re limited to the Fel Hammer ship. (You get sent there pretty early on in the Class Hall campaign, which you’ll get to if you keep following the Legion story.)
If you haven’t finished the starting zone, but accepted a summon you might be able to use your hearthstone to get back. There are multiple inns in the starting area, but you loose access once you’ve finished the quests. You can get back to the portal area (where you have the portal to Stormwind after finishing the quests) through a portal near the embassy in Stormwind.