What is a compound sentence? Well, a compound sentence is sentence that is connected by two or more clauses, comparing, contrasting or speaking about or of things.
The ways you can write a compound sentence are putting two things together (that makes sense) with a comma, if you are talking about John who likes cookies, you can't say Rebecca likes swimming since it wouldn't really make a lot of sense..but you could say Rebecca likes brownies more than cookies, since that contrasts well!
An example of a compound sentence is... I drink tea often, but my friend drinks coffee more.
Since those two things both talk about drinks, it would make a compound sentence, but you could also contrast things, like I really like coffee, but my friend doesn't. That is also a compound sentence.
If you use correct capitalization and go over your spelling you should do just fine!
Also make sure to use correct punctuation to make your sentences stronger!