One observation here from someone who has taken a mindfulness based stress reduction (MBSR) class but isn't currently meditating (but would like to get back to it!):
Expect to get distracted a lot, like your mind wandering continuously. The key here is to observe that happening, and gently bring yourself back to your breath. Don't let it frustrate or discourage you. Watching yourself become distracted and gently bringing yourself back is part of the process of meditating, not some sign that you're failing at it. The noticing of the distraction is in itself mindfulness.
As Jon Kabat-Zinn suggests, you need to approach this with non-judgmental moment-to-moment awareness.
I'd second this: the way you deal with distraction isn't stopping it, it's calmly returning to the object of focus after realizing you've been distracted. This is a 'rep' if you want to compare meditation to training a muscle. In time you get distracted less, but it's not because you 'stop yourself' from getting distracted, it's because in time your tendency to not getting distracted develops through meditation practice.
Expect to get distracted a lot, like your mind wandering continuously. The key here is to observe that happening, and gently bring yourself back to your breath. Don't let it frustrate or discourage you. Watching yourself become distracted and gently bringing yourself back is part of the process of meditating, not some sign that you're failing at it. The noticing of the distraction is in itself mindfulness.
As Jon Kabat-Zinn suggests, you need to approach this with non-judgmental moment-to-moment awareness.