No, you don't, but it is important. Bad grammar or the lack of it should not hold you back. I have known grammar aficionados who write terrible prose. I have also known people who have no grammar write great stories, stories that are direct from the heart. Here is one such writing by Bill Neidjie (Kakadu Man), an Australian Aboriginal elder:

My people… all dead We only got a few left… that’s all, not many.
We getting too old. Young people… I don’t know if they can hang on to this story… might be you can hang on to this story… to this earth…
You got children… grandson. Might be your grandson will get this story… keep going… Hang on like I done My spirit has gone back to my country… my mother.
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     What more could the above say – better grammar would not improve the power of the piece. And, even without grammar it is clear and easy to follow.
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   So do not let your lack of education, specifically a lack of grammar stop you. Rather, feel your writing, to be absorbed into it. If your grammar is not too good, don’t be concerned in the short term – just write. If you love writing, the grammar will come, and you will pick up bits here and there. Whereas, who learnt grammar for grammar’s sake? We learn grammar as a means to an end.
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   If your grammar is suspect, apply the following:
Make sure that the full grammar settings in your word processing software are on (checked). Usually, the default settings only make use of about a quarter of the options.
   When you spell-check, take your time, and make sure you take heed of all those prompts.
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