In short: ANYTHING!
Meditation doesn't have to be sitting with your legs crossed chanting "om" if that's not what works with you. You don't have to "empty" your mind if that just feels like an impossible task,
meditation can be used as a tool to help focus your mind in a certain direction, help you develop your thoughts or sort out your thoughts on a specific area of your life i.e: love, money, sex, relationships, career, moving home or even whether to expand your family.
If you know what it is you want to try and find some answers for, or you want to try and make a decision, then write that down and focus your energy on that word, allowing it to become all you think about (if this sounds difficult - give it a try, meditation becomes a little easier with something physical to look at) think about the words that you have written, and allow your brain to go off on related tangents. Meditation with deep breathing etc, will slow your heart rate and oxygen consumption which will allow your brain to use areas that it may not use daily.
If you find it difficult and feel you need to slow your mind down before hand then see my top ten tips for beginners to meditation for tips to slow down your mind and how to set yourself up to meditate.
When you have the hang of meditation, you'll find yourself able to do very quick 5 minute meditations, for instance, calming ones before a big job interview, or ones for during labour to help you control your breathing and help you make more effective choices for you and baby.
Until then, take it slow, set aside 10-15 minutes to allow yourself to not feel rushed. Of that 10-15 minutes you may at first only get 5 minutes worth of focused meditation - but that is ok! In fact, that's great! It's a very firm foundation that will allow you to build on it and help you get the hang of it.
Of course there are tonnes of other benefits to meditating, such as the affects meditation has on your health, it can help to lower blood pressure by removing stress for instance, but there are other benefits too.
And if you don't necessarily have one thing YOU want to meditate on but there is a general area or theme, then see look in the A-Z and see if you can find a theme that suits you right now.
Of course if you get used to the bits above you can find your own themes and bits to help you meditate too!
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Cause I know my weakness, know my voice,
Now I believe in grace and choice,
And I know perhaps my heart is farce,
But I’ll be born without a mask
~ Babel, Mumford & Sons