Canon Rebel XT EOS with a EF 50mm lense, need to upgrade but to what?

I have a Canon Rebel XT EOS with a EF 50mm lense. I am wanting to upgrade my lense to a 70-200mm but it is too expensive for me right now. What do you suggest I should go to next? I do family, senior & wedding photography. I am looking at a Canon - 75-300mm f/4-5.6 III Zoom Lens on Amazon but sadly to say I don't know much about all the info...