Writing recipes for beer is a lot like writing recipes for food. You need to have a solid understanding of all the ingredients and processes you're working with. You don't start out learning to cook by making lasagna from scratch with no cookbook- you find a recipe for lasagna written by an authority, and try to emulate that until you have it down.
For now, use other people's recipes, preferably ones where you can confirm the original brewer knew what they were doing. For example: recipes from well-regarded posters to HBT, recipes from the kits on northernbrewer.com (every kit they sell has a downloadable info sheet that has the whole recipe), and clone recipes from publications like Brew Your Own (BYO). Once you've been brewing enough, you'll know where you can make small tweaks to existing recipes to make them more to your liking. Once you've done that enough, then you'll have an idea how different ingredients in different quantities come together to make a beer, and should be able to reasonably imagine how a beer will turn out before you brew it.