You're learning a language and have decided to put it to real use. What better way to practice then to converse with someone learning your native language? So you've created an account on the Mixxer or another site, and now you're wondering how to get a language partner. Now what?
First, check your own profile. (My Account -> Edit -> Main Profile on the Mixxer). Does it have a picture? It doesn't have to be of you. A scenic picture of your country, a friendly pet, it doesn't really matter too much. Any picture will improve your odds. Second, did you explain in your comments or description why you're learning the language? This will help others decide if you're serious. If you don't take the time to explain why you're here, others are less likely to take the time to send a message.
Next, if you're serious about practicing, you're probably going to want more than one partner. Everyone is busy, and people's motivation and availability is going to vary over time. I'd recommend trying to arrange exchanges with two to four people and then see how it goes with each of them.
Now it's time to send messages. On the Mixxer, the Profile Search is to the left. Select the languages and then there are two other options, Country and Age. Most users seem to only search for people their own age. Especially if you're younger, I'd reconsider. The fear seems to be that there won't be anything to discuss. I doubt that's true. You know almost nothing about their life, country and culture. It shouldn't be hard to think of questions and find an interesting topic. It's also good practice to speak with all kinds of people. Finally, if you're serious about learning, it's my experience that older users on average make for more reliable partners.
So you've browsed the profiles, see a potential partner and click the button to send a message, but what should you write? It's up to you, but every introductory message should include these things:
A friendly greeting
Your interest in learning their native language and why
Willingness to help them learn your native language
Your Skype name along with days and times when you're usually available. Be sure to include the time difference if there is one. Check http://www.timeanddate.com/worldclock/ if you're not sure.
How many people should you contact? This site limits you to 20 messages per day, but that's only so I can catch spam bots before they send their junk to every user on the site. You should send messages until you have found two or three people with whom you've agreed on a time to speak with via Skype. I usually suggest writing 5-10 messages the first day and then doing so again four or five days later. Keep going until you have a comfortably full schedule, whatever that may be for you.
Good luck. Feel free to leave any other tips in the comments below.