A word gets into the dictionary by being used. Lexicographers - the people who write dictionary entries - don't make words, they find them and record them.
When modern lexicographers define words or find words to add to dictionaries, they tend to approach their work from the angle of descriptivism. That is, they observe how the language is being used, and then write definitions based on that research.
Lexicographers also try to imagine what words people actually want to look up. There are two important factors to keep in mind here: 1) Is the word in widespread usage? 2) Does the word have staying power?
So, you can't invent a word and petition to have it admitted into the dictionary. You can invent a word and use it, of course, and if your word catches on, it might end up in the dictionary some day!