Just a couple blog customization questions

Just a couple blog customization questions:

1. How do I customize the single post pages?

2. I took out displaying the Featured Image and now have a huge white space at the top of each post between the post content and title.

3. Also, on the main blog page that lists all the current posts, some of the the featured images are blurry while others are not.

4. How do I add Previous Post and Next Post options on the bottom of each post?

5. Lastly, the author's avatar is not showing up. How do I fix that?

I cannot think of anything else right at this moment but I feel like there were a couple more questions.

Thanks!