Just read
Mindfulness in Plain English. I've read it like seventeen times, it's a really good book and the first one I'd recommend to beginners.
As Bel Marduk said, it's not about 'clearing your head'. Or at least, that's not something you just do. The closest thing to that which you will be doing is adopting an attitude of renunciation. Meditation is giving things away until you're left with nothing but awareness of the present moment. It's not a 'doing', or if it is it's a kind of 'doing by non-doing'. You just let go, let go of your expectations, or looking for signs of progress, all of your daily concerns, even your sense of identity, all of these things are just concepts. Your aim is to look past the concepts that normally rule our lives to see the bare awareness underneath them, this non-conceptual awareness is 'mindfulness'. In every moment we experience there is some portion of mindfulness (and attention) but in the case of meditation we're working to exercise these faculties.
You will get distracted by thoughts a lot. The other aspect of mindfulness is 'remembering', essentially remembering your intention to cultivate bare awareness. Mindfulness will alert you to when you're getting distracted, and bring you back when you've gone off track. But you can (and ideally should) be mindful of everything. Mindfulness is like a mirror, it impartially reflects everything you put in front of it while it is there, and holds on to none of these things when they leave. Mindfulness is the most helpful tool in defeating all of the obstacles a meditator faces. Say if you just can't get your mind to calm down, it seems to be running everywhere and you're getting frustrated, then just watch your mind run to and fro, and watch your frustration. With that these things might suddenly go away, or they might stick around for many years, but regardless you will be gradually loosening the grip that these things have on you from observing directly their causes and conditions, causing the mind to naturally become disenchanted with them and see past them.
The other aspect is concentration, or essentially narrowing your awareness on one item. But this isn't done by striving, rather the development of tranquility causes this to happen almost effortlessly. With deeper concentration you will naturally be distracted less, this is also the faculty that lends intensity to meditation. You can equate concentration to light waves focused by a magnifying glass. Ordinarily an amount of sunlight might fall on a piece of paper and only heat it slightly, but if the same amount of light is brought together to one point, the paper bursts into flames. All meditation employs both mindfulness and concentration to some degree, only one or the other might be emphasized at different times.
Meditation is essentially just a form of mental exercise where you're attempting to strengthen the faculties of mindfulness and concentration. Both can be considered forms of attention in a sense, just that mindfulness is a broad and inclusive kind limited to phenomenon in the present moment and thus enables us to see insight into phenomenon, and concentration collects awareness on a single point and in the process generates great intensity.
