This should be an exceedingly obvious tip, however it's the most crucial to actually being cast. The producers of "Biggest Loser" are not looking for someone who needs to lose 10 lbs. to 15 lbs. of weight. They want people who are morbidly obese and running out of options. They typically want people who may have to consider gastric bypass if nothing works for them. Any prospective constant must be honest about their need to lose weight. It can be hard to face up to the fact that one needs help, but it's the first step.
Often when admitting that weight has gotten out of control, people become shy and pull away from the world. However, no reality producer is looking for someone who pulls away from interaction. They need people who will give good answers when asked interview questions and engage with the other contestants. Spend some time in front of the mirror practicing being personable. it is the first step toward being cast.
Being dramatic is key because the producers want someone who will create conflict. Reality TV became popular because people were doing unbelievable things. Be prepared to answer questions about the most embarrassing life moments and make sure the answers are fun and interesting. Producers ask the questions, not because they care about the specific story (everyone has a sob story they can tell,and one story isn't more interesting than another); what they are looking for is a character who can tell an interesting story. This is another key element in finding out if someone is TV-ready, can they tell an engaging story and get the audience interested in them.
If all of the above criteria fits and is comfortable, visit the casting website, download the application, find the nearest casting site and make a video. The video is another tool to prove that a person can be interesting on camera. It's not about production quality or the content. It's all about the subject. Be sure the application is complete and interesting. Keep the video short and sweet and make sure it conforms to all requirements. Attend the open casting call and fulfill any additional requirements put out by the show.