Read all sorts of different stuff. Obviously any sort of fiction or non-fiction, but also things like newspaper articles, encyclopedia entries, product manuals, etc. Hell, even food packages will sometimes have short blurbs about the product or the company that made them. Reading stuff from a wide variety of sources will give you an idea of the many different ways words can come together, whether to describe a vacation spot, explain what the gear settings on your car are for, tell the story of a British spy, and so on.
I Googled "writing ideas" and found this site with a lot of writing prompts: http://thewritesource.com/writing_topics/
Yeah, you're obviously past first grade, but if your writing skills are in need of some work then it might help to pick some of the easier prompts. Do some searches of your own for more writing ideas or tips for good writing.
Buy a composition notebook (the ones with the marbled, thick-ish covers and the pages that aren't perforated for tearing out) and use it to write in; they're fairly durable and can be written on your lap if there isn't a table nearby. Write in your living room, write while you're taking a shit, go to a park or a library or a restaurant or anywhere you can sit, and write there. Maybe jot down some writing prompts to write about while you're out, or make up a story about the place you're in or some people you see. You could even just describe your surroundings.
Say you're at a restaurant. Describe the various sounds and smells. Look at the people at other tables; what are they wearing? Do they look happy, frustrated, bored, etc.? If you've been there before, do you have any memories of previous visits? Write down what you order and describe it when you get it: how it looks, how it smells, what it tastes like, what you do/don't like about it. What kind of mood is your server in? If they're upset, trying writing out some ideas why they're upset; they don't have to be right, just whatever you think of.
Most imortantly, do not get discouraged if you don't like what you're writing
. I would consider myself a pretty good writer, and I write stuff that I look at later and just go "that was fucking terrible" all the time. It takes practice and a lot of trial-and-error, but if you keep at it then you'll slowly but surely improve.
If you write something you don't like, think about why you don't like it. Even better, write
about why you didn't like it! Thinking about your writing and commenting on it is a good way to both work on your critical thinking skills and improve your writing as a whole. Choose some things you didn't like, write what they were and why you didn't like them, and think of some ways to do it differently next time. But also choose some things you did like and write why you liked them. It's important to not only identify different areas for you to work on, but to also reinforce the stuff that you feel you did well.
Well, that was quite a bit longer than I thought it would be. But I really hope it gives you some good tips. Remember, it may take a good while before you think you're writing is good. But just keep writing! You'll never get better if you don't practice, but if you do practice then you will get better. Hope this all helps, and good luck with your writing!