if you're talking about ON page SEO, then you wanna cover your basics. choosing a keyword to concentrate the page on, have the keyword in title, stuff of the sort. and i'm not a twitter user, but i assume a re-tweet is when you just "tweet" the same "tweet" as someone else.
Keyword research is the first step of the OnPage optimization. And when you are repeating you tweet that is re-tweet. I can't understand your third question.
hmm so i assume you are a beginner so i suggest to read google seo starter guide.. it really helps.. and about retweet? yah its for twitter.. re-tweet means repeating tweet..
keyword research & analysis and content analysis are the main point to do when you want to start On Page.
First Keyword research and after add Meta tag in your website after optimize content and add H1 tag with your targeted keyword ....