Are there certain sounds or letters you have more or less difficulty saying? It helped me in school to kind of script my presentations so they were words I could say mostly fluently. Of course, in the long term it would be better to work on sounds you have trouble saying, but if you're under a lot of pressure it's okay to stick with the "basics". And rehearse a LOT. I took a public speaking class where we had to rehearse our presentations with a partner at least 7-8 times, and it was embarrassing the first few times but I kinda got the hang of it after.