Before attempting to straighten your hair, get it about 80% dry by blow drying with a diffuser, blotting with a towel, or waiting until hair dries naturally. Then use a blow dryer or hair straightener to complete the style. You will need to practice to learn how to use a flat iron properly, but you will eventually get the hang of it.
*It will require a lot of heat if you have extremely curly hair. Always remember to use a shot of cool air to set the locks you have just straightened.