Typically I'll need to know how many words the text has, and when you need it by. It's also helpful to know the target audience and purpose. It's important to know the file format, as some formats can be processed by my translation software, which speeds up the process, while other formats require more manual work.
If you have had something similar translated before, I can use it as reference in order to ensure that the terminology remains consistent. Or maybe you even have a glossary you'd like me to use. No problem. I also like to know if there is any specific style guide you prefer