It all depends on your level of English.  Writing about abstruct concepts for example is even hard for native speakers.  I would propose starting with something simple and see how you feel about it.

For example describe in some detail a wonderfull day you had.  What did you do that day.

If it sounds too long for you just pick a small part of it, like what you did in the moring.

If there was a book that you read you can try to summarise it, if it was not a complex book.

You can try to do something similar for a movie that you liked.

Play around with your thoughs see what you can come up with.  If in doubt post it here and people will guide you.

Good luck.

It is frustrating to write in a new language because we do not have the words yet to make complex compositions. My advice is to think like a child, and try to find the same enjoyment as a child would do. Here's what I do when I first learn a new language:

I stick flashcards on every item in my home. Then I make sentences when I use an item: "This is a refigerator. It is tall. It is white. It is cold. Another word for it is fridge. My fridge has two doors." I speak out loud; it is important for pronunciation practice to hear the words. As I learn more vocabulary the sentences become a little longer: "I open the refrigerator door. Inside are 5 shelves. There is some cheese, a bottle of milk, and six cans of soda. I take out the cheese and cut it. I drink a can of soda."

You can do this as easily by sitting at your computer and picking an item you can see, and describing it. Write it down, but speak out loud anyway. It also helps to remember words better if you write them down in a notebook. There are hundreds of items; you won't run out of subjects any time soon. Do the same thing when you go outside. Take a notebook with you. If you don't want to talk out loud when other people can hear you, take photos. There are a thousand words in every photo. ;) When you see something for which you don't know the word, describe it first and later look the word up when you get home: "It is an animal. It has four legs and a very, very long neck. It is as tall as a two-story building. It has short hair, and brown spots with white in between." Write small compositions for everything and post them here or to lang-8.com for corrections. Find a language partner and tell them about all your things.

Practice is essential. Practice, practice, practice. It will not take long before you can handle more complex composition.