Instead of sending your credits or assets (ships, vehicles, etc.) to the party you're trading with directly, you send your part of the deal to a trusted middleman (such as Togan or Syn, a full list can be found in the Traders Lounge), who also collects the part from the person you were trading with, and then after checking that the items/credits are what they say they are, and are undocked and undamaged, the middleman sends it off to the correct party.
Basically, just send whatever you were going to trade to the trusted middle, rather than the person you are organizing the trade with.
Also if someone offers to middle for you, it does mean that they are a middle, but their reputation is considerable. Please take a look at this thread to see reputable middles who will not steal. http://www.swcombine.com/forum/thread.php?thread=9177&page=0
To clarify a bit on what I think Maziel was trying to say - if someone says they will middle, it does NOT necessarily mean that they are trustworthy. Finding trustworthy middles that you like to deal with is personal preference over time. Since you are new, stick to the very well-known community middles until you get a better grasp for who in else in the trading community you are willing to trust your credits with.