Tips for Writing in Japanese
Learning how to write in Japanese can be pretty tricky. Even if you have been reading a lot of Japanese and have a feel for the written language, it can still be difficult to actually put together what you know to form your own sentences. These general ideas can help you figure things out.
I highly recommend that you check out Lang-8 to help you with your writing.
1. Do not try to translate.
When I first began to write in Japanese on Lang-8 I would write out a sentence in English and then try to translate that into Japanese. It was impossible. My knowledge of English far outstripped my knowledge of Japanese, so attempting to match Japanese to the English was just not going to work. So I flipped it. I would write my entries in Japanese first, and then translate them into English. The people on Lang-8 that are reading posts are interested in English as well, so by giving them my entry in both languages they could more easily see what my mistakes were. Furthermore, by writing in Japanese first I was able to fully utilize my knowledge of the language and then bring down my English to match - instead of trying to force my Japanese to a level it wasn't at yet.
2. Do a Google search for your sentence.
One great thing about the internet is that almost any thought has already been posted by someone else. Not literally every detail, but the same general format or theme. So you can take any sentence that you write and feel uneasy about and put it into Google, then see what comes up. If there are no coherent results for your sentence, then you may have some grammatical errors. You can then try to narrow the problem down by selecting parts of the sentence and testing to see if you get results. I still use this method when typing out messages in Japanese, just to make sure that I am recalling what I think I am. You can use a dictionary and Anki to make sentence cards this way as well.
3. Have someone proofread for you.
This is one of the most difficult things to achieve - finding someone you can trust to give you an honest evaluation of your Japanese. If you are in a Japanese class, then you have a teacher or professor that can check your writing for you. If you aren't in a class, then you can use Lang-8 to find someone. In the past when I have used Lang-8 I always had multiple people review each of my entries, which means several different opinions for each entry and how to improve it. The important thing to remember is that these people are not being paid or doing this professionally, so they won't find and correct every single error - but they are nevertheless a boon to your studies.
4. Start simple.
At first you may be tempted to write out a complex thought or write about the entirety of your day, but it might be beyond your ability to do so. If you haven't attempted to write Japanese before and you are relying on feedback from others to improve, then start with something good and simple. Write out five sentences about your day. Try to vary up sentence structure if possible. Look through manga for examples and try to copy the structures. Now try to adapt that sentence structure into your own thought. By posting simple, standalone sentences in the beginning (along with an English translation of what you were trying to express) you will likely get good feedback on what you've done right/wrong.