A good question, no? How can one possibly pray if you don’t know how to pray?
A traditional Chasidic story speaks glowingly of the prayer of an uneducated Jew who wanted to pray, did not speak Hebrew, but thought Hebrew a necessity for prayers to be heard by God. So he began to recite the only Hebrew he knew: the alphabet. He recited it over and over again, until a rabbi asked what he was doing. The man told the rabbi, "The Holy One, Blessed is He, knows what is in my heart. I will give Him the letters, and He can put the words together."
A Prayer for Every Need, a book by old faithful Norman Vincent Peale, provides wording for prayers for many situations. If you’re one who finds a formula or model helpful, you can download a free copy of that book here.
Charismatics would answer that you pray in your ‘prayer language’, the mysterious gift of praying in tongues many of us have heard of or experienced.
Charisma magazine published an interesting article exploring many aspects of this, Tongues: Is It the Initial Sign of the Holy Spirit's Filling? by John Sherrill.
Personally, I’ve found ‘HELP!’ and ‘Thank-You’ two of the easiest and best.