Hello bex and Welcome to History Help!
Since you are new here I guess you didn't realise that your first post would be queued for 'approval'. I've done that now and have also deleted your duplicate message.
I'm afraid we won't "do your work for you" (as we say in the Forum Rules - see link at top of the main page), but are pleased to offer some guidance.
To write a good speech (for which ever 'side' you choose) you need first of all to understand what the Republicans' and the Democrats' policies and attitudes were in the 1920s. Have you not studied that in your lessons? Perhaps you have a textbook for your GCSE course? Try looking through class notes or the textbook for information.
If you are stuck for basic info. then try this site:
sorry the link I want to give isn't working at the moment. I'll post again when the site is back up
This section of the BBC Bitesize Revision site is quite helpful.
Once you have the basic information clear in your mind then (when you have chosen which Party to represent) then decide on a number of key points (say 3-5) and deal with each one in turn.
If you are being a Republican you could, just as an example, focus on
selected sections of the American economy which have been successful whilst 'you' have been in power. If you are a Democrat you could, by contrast, attack the Republicans for their refusal to support industries which were struggling at the time.
I hope this enough to set you off on the right track - but as I said at the beginning this is not a 'cheat site'
If you need any specific help understanding a particular point then do post again.