I'd like to know how to start off my main body of discussion and carry on throughout the speech with a strong argument. The speech it to go for 3 minutes. Thank you!
Why don't you start with a personal anecdote from your childhood? For example, you could say that when you were young you always brought home stray animals to take care of. You could then launch your speech about why you want to be a veterinarian. The idea is to personalize the speech as much as possible, and make it interesting. Focus less on the money you would make and more on the personal satisfaction you'd get from the job.
I'm always a big fan of speeches that tell personal stories. An alternate idea to the above suggestions is to make your speech all about you.
As far as the topic "Dream Job" is concerned, it can be yours. I'd give a personal story behind why your dream job is your dream job. In this way, your argument is completely personal (which means it cannot be wrong or argued against) and largely based on feelings and emotions, which are usually easy to define when you are passionate about something.
I like the idea of using former speeches as a guide. I think that this might be a good technique. Overall, I would think that you need to have a very strong "hook" that pulls the reader into your speech. Perhaps, this can include an interactive tone, where you ask the audience to define their own dream job and work from there. If you are assessing the difference between one's dream job and one's "real job," perhaps striking a more reflective tone could be appropriate. It might be good to assess where the speech is going and carve out an introduction that is effective in articulating it. In terms of the "strong argument," you have to figure out what this is and make sure you are articulating it in different ways so that it resonates with the listener. There is little else anyone can give you outside of this and not knowing what you are attempting to prove.
There are several approaches that you might take in structuring your speech. You might start with a vivid description of a typical day in your dream job. Give enough detail so that your audience can picture what that day is like, as if they are experiencing it vicariously. Then explain the various reasons that it is appealing.
Another approach is to describe what a dream job is not. Start with several descriptions of jobs that others might consider to be dream jobs and explain why they are not fulfilling to you. Then describe the job you do find fulfilling and explain why.
Another approach is to find facts and figures about your dream job--the demand, the pay, the conditions, the necessary skills, the experience required, the intrinsic value--and explain why these particular characteristics fit your goals.
Lastly, you might use an anecdotal approach. You might describe the person who inspired you or mentored you. Or, you might describe the situation that led to your epiphany that this was the job that you wanted to do.
And, you might even use a combination of these. The major advice that I can give you is keep it real by making sure that what you say is meaningful to you. If you are enthusiastic and authentic, your audience will sense your engagement and respond positively.
I hope these ideas helped.
There are a number of ways you might approach this, one way to help get you started would be to find some good speeches, regardless of the topic, and see how they carry their main ideas or thoughts through the whole of the speech.
As you begin to examine your own topic, you might look for a common thread that runs through how you came to feel about your dream job or what it is about your dream job that makes it so, well, dreamy.
You can also get more technical and look up some rhetorical techniques, they are found all over the place online or in books, recently there are loads of books published for the AP Language and Composition class that are all about rhetoric. These can help give you some structures to go along with your great ideas.