My comments are assuming you are U.S. based.
Get clients through friends, co-workers, previous employers and by going out and meeting people and companies. Start a blog or contribute to open source projects etc. That always helps your credibility and also helps you get recognized over time, but it doesn't happen overnight.
1. In the U.S. most contract placement agencies will charge between $75 and $135/hr per person, except in some of the higher cost locals where it will be more. You are competing against them regardless of what people say, simply because companies will talk to these agencies when looking at projects only requiring one or two people to do over a short period. You compete against them not by lowering your rate, but by offering a solution not a warm body. Overall, I see rates typically from $100-$185/hr in the South East, some try to get $225-250 but reality is I see those people losing deals and/or constantly discounting their rate to get work. Your rate should constantly be going up as you get more experience and successful projects behind you that you can point to, but you'll find the point which it is in balance with the market. Also, discounting your rate is generally a bad idea, don't do it if you can avoid it. A couple of clients have told me before that they think a discounted rate means you know you are overpriced and are just looking for a sucker. Not sure I totally agree, but I don't discount my rate because I never really have a fixed rate, I will vary it by the job, duration etc.
2. Avoid these sites, unless you are outside the US in a low cost nation where you can live on $1-2k USD/month. Use friends, co-workers and previous employers as a first start and go out and talk to people and companies. We have found work on Craigslist a number of times where companies were looking for a "coder" to write their project. I don't get turned off by poorly written ads, or obviously non-technical people seeking help and will call them up and discuss their needs. I have walked away with $40-70k deals this way when some friends looked at the same ad and said it was a waste of their time to call. Fuck it, call them the worse you find out is the job isn't worth your time but you can still use practice on your sales pitch and interviewing potential clients. I have a client right now that I found on Craigslist and called up, that contract ended up being roughly $60k over the next few months, not bad for less than an hours work searching craigslist and calling him up. Of course it took a couple of weeks to get the contract in place etc, but well worth my time.
3. There are a ton of things you need to know, get yourself insured, always have a contract with clients, get paid by the hour, day or week. Demand payments be timely and stop working when they aren't. Small clients I make pay weekly or at worst NET 10, medium clients I go NET 10 and up to NET 30 the larger they get. Accept you will get screwed over a few times with clients not paying or paying slow, just learn from it and try not to repeat those mistakes. Set expectations clearly, and avoid clients you feel are going to have unrealistic expectations, no matter how desperate you are. Don't take on more then you can do, it will just piss people off (rightfully), at the same time, don't turn down work, just schedule it for the future when possible. Don't let small clients drag out the project or let scope creep in. Every client will try, but small businesses seem to be the worst, they will call/email you constantly and always want more for free. Don't let people devalue your time by getting your time for free, you don't have to be rude, but make them respect that your time is worth money.