It would have been helpful if you had given us more information, like do you know the coordinates of your portal? Do you have at least 10 Obsidian on you? Do you have, or have the resources to build a chest or Ender chest?
If you're not equipped with anything particularly valuable, just jump in some lava. You will respawn in the overworld at your last spawn point (or your original spawn, if your last one is no longer valid).
If you know the coordinates of your portal, but simply can't find a path back to it (that's what happened to me), find the highest point you can from your current location and pillar up into the ceiling. Dig a path up to the Bedrock roof of the Nether, and then a few blocks below that, start tunneling back to the X/Z coordinates of your portal. Be careful to avoid lava. It's less plentiful up near the top, but you still find the occasional source block.
If you have a chest, hit F3 and take a screen shot of your coordinates, put your stuff in the chest, then kill yourself. Then bring a lot of torches and cobblestone to mark your path on your return to the Nether.
If you have obsidian, but nothing to light a portal with, you could try to get a Ghast or Blaze to shoot it with a fireball, or you can gather the needed resources (all available in the Nether) to make a Fire Charge.
If you don't know the coordinates of your portal, don't have any chests or obsidian, and are carrying a lot of valuable stuff, then you are pretty much screwed. Your options are extremely limited.
(1) Note your coordinates, drop all your stuff, jump in lava, and prey you can make it back to those coordinates before it all despawns (unlikely).