Questions are generally only deleted by moderators (diamonds or high-rep users) when they actively cause harm to the site itself. Examples are spam, racism and other discriminatory behavior), intentional misconduct, and of course anything that might be considered fun. We hate fun. Causing minor harm to a user's reputation is not a reason why a delete vote would be cast by us. Any user can delete their own post at any time for any reason.
As for how to prevent downvotes in general:
- Make your questions clear and answerable
- Make your answers clear and on-topic
- Heed the feedback of other users suggesting you clarify, reword, or even remove parts of your post
- When asking a question, don't post right away - read it after 20-30 minutes as if you've never seen it before. I personally find this answers over half the questions I type and then I don't even need to post them.
- Edit, edit, edit!! Editing your posts and always making them better is the number one way to increase signal to noise ratio which is where reptuation comes from.
- Ask questions here on Meta when unsure of policies, procedures, and customs (which you've clearly started doing already)