Take a lesson online. Do your research by exploring a number of sites and find one you feel you can learn from. Begin with a basic site that doesn't charge to see how well you learn.
Register for a class. If your local college or community college doesn't teach Chinese you can usually find a class offered in a community center.
Find a self-guided lesson book. You can buy them online but your local bookstore might carry a copy you can review before you purchase.
Review with audio. You can listen to podcast or audio CD on your way to work or while jogging. Audio lessons are especially good for teaching pronunciation.
Network with other learners. They can offer non-judgmental feedback and tips that helped them learn better.
Practice, practice, practice. Find any opportunity to expose yourself to Mandarin. Make conversations with native Mandarin speakers, read Chinese newspapers and comics and watch Mandarin movies (with and without subtitles).
Visit China. You can go with a university or community tour group or you can take a job teaching English. Nothing helps improve your language skills better than immersion in a language and its culture.