How do I choose a forum to post a question on?

You formulate your question to include keywords relevant to the topic to help your question land in the appropriate subcategory, when you Submit it. Some users even preface their question with one or more words from the subcategory heading (where they want it to land) such as "Polls: <poll or survey question>" or "Dogs: <dog related question>". 

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IF your question doesn't land in an appropriate subcategory when you submit it, you may relocate it yourself. Moving your question can be done by clicking Edit (pencil icon for mobile)>Edit category just beneath your question, then selecting the category that's appropriate for the topic.

by reading the rules of the forums.

If you mean category on Y!A, the algorithm will pick one for you. If it picks the wrong one, click " edit" and then "edit category"

you cant before the question is posted after its posted you can pick it before you post it yahoo will try and guess what category to place it in based on key words used in your question if its not placed correctly you can move it to move it go to your question under the blue edit menu click on edit category then select the new category to put it in then click submit