Rejection slips are part of most writer's lives. People like Stephen King and J. K. Rowling received plenty of them before they were published. For most aspiring writers this is a part of life.
I have queried several literary agencies in search of an agent to represent my manuscript/novel "The Awakening". I have gotten several rejections and I am still waiting to hear back from a couple of others. I have learned a few things along the way.
I bought a book titled "Guide to literary agents". It not only lists agents and agencies, but it offers a lot of helpful hints. I recommend it but don't use it exclusively.
I began on the first page of the list of literary agents. I checked under the heading of each agency to see if they were accepting manuscripts for the genre that I am writing in. In this case I looked for those who accepted science fiction stories. Then I look up the agency on the internet. Sometimes you will find differences between the book and the website but it's usually pretty close. Definitely go by the website. It should be the most up to date.
The book tells you how to submit to these agencies, but it is a general recommendation. All the agencies I have come across tell you what they want and how to submit your material. Very rarely will an agency accept snail mail. They want it electronically. And I can't imagine sending it any other way.
A few agencies want you to send your material through their email often requesting that it be incorporated in the body of the email. That's fairly easy. It's just a matter of copying and pasting. Then there's the other way which it seems that more agencies are gravitating to. They have a portal. You simply fill in the spaces with the information requested and hit "submit". I wish all of them were like that.
What the agencies are requesting are pretty much the same. This refers mostly to fiction. To start with they want your name, the title of your book, and the word count. They require a query letter. This is to introduce yourself. Some of them ask for specific information, such as experience. A synopsis is often required. It usually consists of one or two pages but go with what they specifically ask for. Sample pages are also required. They can range from a few pages to several chapters. Some will ask you to name a book that's similar to yours and how your book is different.
My advice is to follow their directions as close as possible. These agents get a ton of submissions, and you wouldn't want to get turned down on a simple technicality.