Common sense is the most essential aspect necessary for a safe trade. Since you're new, most gold sellers won't dare go first to you, so I highly suggest you use an OMM, or at least a free trusted one (there are a bunch out there). Since you're only looking to buy RS items, your trades will be much safer than account trading, so don't worry too much, so long as you get the items you desire in the end.
1) Get a gold/item seller and ask him if he wants to trade. Once you've agreed on all the terms, proceed to the next step.
2) Contact an OMM and bring him into the conversation on MSN.
3) Do as the OMM tells you to, and once you're done, you may vouch for each other.
Shortened it, but the process is a bit longer than it looks like on paper. Depends on how efficient the seller really is, to be honest.
Good luck.