Making that first sale is far beyond exciting and the first time the writer must seriously think about legalities. Of course, future sales are just as exciting.
The first sale of a writing project may be just a verbal agreement or an agreement in an email. Even then, though, the writer must learn how to protect yourself and what you have agreed to provide and receive.
If the first sale results in the writer receiving a written contract, it is important that you understand what is included in the contract. Do not let your excitement over a sale keep you from knowing your rights and what you are agreeing to with a publisher.
My article on Understanding Contracts contains a brief bulleted list of things of which you need to be aware. The most important part of the article this time are the links to a number of online sources for excellent discussions about contracts. For my full article, see the Writers Tools page and the Finished Products section, Publishing Process. Or use this direct link to the article Publishing Process: Understanding Contracts.