Prayers come in many different forms, and different people may have better success with some forms over others. If you're having trouble just jumping right into personal prayer, you might want to try something else first... For example, you're familiar with prayers that involve repetition (doing so many sets of our father, for example), and that's certainly a valid way to pray that is used in many different religions. Repetition of a particular prayer like that puts you into a meditative and receptive state, which can allow you to commune with deity. Using prayer beads can be similar.
There is also reciting (but not repeating multiple times at once) an existing prayer, either one you've written, or one written by someone else. A lot of Greek pagans are fond of reciting the Homeric or Orphic hymns, for example. After reciting a hymn one might then say some personal prayers, or just have a moment of silence. Opening with the same prayer before personal prayers is another way to get into the 'right' frame of mind, acting sort of like an icebreaker, almost. Do it enough and your mind goes, okay, yeah, it's time to pray, and then going from reciting a prayer into personal prayer time becomes easier. Some accomplish the same thing by sitting at their altar and lighting their candles/incense, or whatever else, first. Eventually you might find it easier just to get right into personal prayer without these aids, because you've simply become more comfortable with it over time.
There's a book called "A Book of Pagan Prayer" by Ceisiwr Serith you might like to check out. It's a prayer book with a lot of different examples in it, but it also covers the whole how/why/etc of prayer different ways to pray, and all that.
I sing of you, blessed, night-winged Dream, Messenger of things to come, greatest prophet to mortals, in the quiet of sweet sleep you come silently and speak to the soul.