A good exercise that you can do while doing you're actually writing is to leave the dialog tags out, and see how easy it is to follow. When it gets hard to track, add the dialog tag, but before you do, try to reword the line so that the character's voice is more easily distinguishable. If that works, you may not need the tag. And, tag or not, making the characters' voices distinct makes the story a lot better.
Also, for formatting, if the same speaker says more than one paragraph, the end quote goes only at the end of the last paragraph. For instance:
John said, "This here is a paragraph that I am saying. It's a lot longer than it needs to be so you can see the line wrap and know that it is actually a paragraph, just to make the example more clear.
"This is a second paragraph. Notice that I didn't stop speaking after the previous one, so there is no end quote there. It does go here, because I'll shut up now."
And yes, even single lines of dialog need a space after them if the start of a paragraph is not indented.
Also, for formatting, if the same speaker says more than one paragraph, the end quote goes only at the end of the last paragraph. For instance:
John said, "This here is a paragraph that I am saying. It's a lot longer than it needs to be so you can see the line wrap and know that it is actually a paragraph, just to make the example more clear.
"This is a second paragraph. Notice that I didn't stop speaking after the previous one, so there is no end quote there. It does go here, because I'll shut up now."
And yes, even single lines of dialog need a space after them if the start of a paragraph is not indented.