Upon clicking either of the 'Post' links you will see a page like this:

Subject: It is strongly recomended that you include something like 'Glasgow to London' (example) as your subject because this is the line that will appear on the main Lift Share page.
Message Icon: Play with this or ignore it as it pleases you, it's not important..
Add BBC Tags: (See above)
Message: This is where you write all the info you feel you need to include in your journey post! You may wish to suggest sharing fuel costs, or perhaps a relevant stop off point on multiple legs of a journey, etc and so forth. Be aware that if you are a registered member of the forum you need not include any personal contact details.
If you do not wish to register, you will have to leave some form of contact details in this section. I recomend you take the simple and free steps to register on the forum. (If you're not sure what any of that means, you can read more about it here).

Yep: That's it, you're done, just click 'Post'. Nice.