Alecia,
If you have done this soft of work before turning to freelancing, the need for defining the expectations of you and your clients will be pretty much the same. If you have done in the real world the type of writing you expect to do for clients on Upwork, you might have a steep learning curve in learning how to clearly define each project's parameters, the timing of your work and the price you can successfully charge.
Use the Upwork message board for each project to put all agreements with the client in writing. Even if you exchange information using regular email, document the important items and add documents via the Upwork message board. To be on the safe side, always assume that someone (the client or Upwork) will question what you agreed to with the client, and nothing beats having it all in writing here on Upwork.
It is my understanding that if the client initially sets up a contract as either hourly or fixed price, and the two of you agree the contract needs to be of the other type, the client will need to create a new project with the correct type. (Others here can confirm or correct that statement.)
Be sure to install and use the Upwork's Time Tracker software to track ALL work time you exect to be paid for on hourly projects. DO NOT add manual time, even if a client says it's OK to do so. Clients have no obligation to pay for manual work time and can ignore paying for it for any reason, or no reason at all.
Good luck.
