I wish we had a set of instructions for joining a clan stickied at the top of this forum, because while posting here seems logical and lets folks know of your intent, the marketplace is too broad.
With that, I present:
Tripper's Guide to Getting Clanned Up
1. Become proficient at the game. Not expert, not "1337", proficient.
2. Find the clan servers on your continent that give you a workable ping. There's no sense in joining a clan when you can't play smoothly on your home server.
3. When you've found a good set of servers, play on them. For months. Communicate using VOIP and if the clan has a TeamSpeak server, get on it. Become a familiar face and voice and show your teamwork skills. Call out enemy positions and take orders with good grace. If you find yourself on the opposite side of prospective clans, be the most determined enemy they've ever seen.
4. Add the clan tags of your prospective clans to your Friends tab in RO, followed by the wild card asterisk (*). For example, (BB)*. This will show you where your prospective clan's members are playing. Again, become familiar.
5. After you are a familiar face, read the clan's admission policies on their websites and see if you fit into what they are looking for. Some clans are strict and disciplinarian, some clans are loose gatherings of friends, and most lie somewhere in between. Make sure the clan is right for you.
6. If you do feel that their guidelines are describing you, more or less, then sign on to the clan's forums and introduce yourself.
This takes time and effort, but if you follow these steps you will not be showing up as a stranger at the door, or as a random request on a web page for the entire game community, worldwide. You will be a known entity. You may well already have been noticed, and you will have proved that you have a commitment to the game.
Also, play in the Iron Crescendo campaign if you have the time. There is no better way to meet clan players and show off your teamwork abilities in tense situations. You will become a known entity in the RO community quite quickly.
You will notice that I've said very little about these RO forums. My only advice in this area is that you must realize that every post you write in these forums will be seen by your prospective clans. If you act like an ass in public, most clans will not want you no matter how good a player you are. If you create a cool persona on these forums, however, this can only be seen as a plus to your eventual recruitment.
Good luck, and good hunting!