If you make a portal, you'll enter the Nether. However, you will enter through another person's portal. The game will not make an Nether-side portal for you. If you go back through the Nether-side portal, you may get to yours, or more likely you will go to someone else's.
To link your overworld portal to the Nether-side portal:
1) Stand next to your overworld-side portal. Hit F3. Write down your X and Z coordinates. Oh, and have 10-14 obsidian blocks and your "flint & steel" with you.
2) To plan your Nether-side portal, divide the X by 8 and the Z by 8. Example: your overworld-portal is near (234, -567). You need a Nether-side portal at (29, -72). You can ignore decimals. You don't even have to be very exact.
3) Go through to the Nether. Travel to the Nether-side portal coordinates, (29, -72) in this example. Cobblestone is handy -- unbreakable by a Ghast, and you probably have plenty. Dig through Netherrack and build bridges as needed.
4) Build a portal at the location. Activate it (light it up!). Go through... you should be home!
