What specifically are you unsure of? Do you have a question to answer or something for your presentation? If we know a little more about the context then we can help you out n the details.
I need to present on whether the argument is convincing, but I'm struggling to understand on a basic level what the passage is actually trying to state, or what Plato is trying to state.
The Theaetetus is a dialogue concerning the nature of knowledge (and so truth). Protagoras is being presented here as advancing a sort of relativism, viz. that there is not Truth with a big T, only relative truths with a small t. The effect of this is that 'truth' is different depending on who you ask.
Socrates is trying to force the point that it is incoherent to hold a position that reduces truth to a relativistic opinion rather than a verifiable thing - this is why he brings in the examples of the men at war (slightly prior to the extract you have given).
Protagoras holds opinions that he (Protagoras) believes to be true.
If truth is relative, then a man next to him might say that Protagoras' opinions are false.
Who is right? It seems like Protagoras is right but his objector - according to Protagoras' own doctrine - is also right, meaning that Protagoras' opinion (or doctrine of relativism, if you like) is at once True and False.
Socrates thinks this is undesirable because it is an absurdity (think of the Forms as objective, independent, immutable Truths).
This page will help make sense of the finer details.