Careful and detailed organization is perhaps your most crucial task for a number of reasons:
An oral presentation always moves forward. Your classmates cannot easily go back over material they have failed to grasp, unless they ask you to repeat yourself. It is especially important that you be clear in your exposition of ideas the first time around.
You almost certainly have a time limit on your seminar. Exceeding this limit, which poor organization will likely cause you to do, is usually too disruptive to the class schedule to be allowed. You will find it embarrassing to be cut off before you are finished.
You will probably be nervous. Even if words usually come easily to you, they will not do so during your seminar. Only detailed organization of your presentation will then save you from a halting, chaotic delivery.
Keeping this last point in mind, shouldn't you just prepare an essay text and read it aloud? Well, this might solve the halting delivery problem, but it will likely not make for a very zesty seminar. Unless you have stage experience (or a histrionic personality) you probably don't declaim prose texts with gripping expression. You'll likely project more spontaneity by extemporizing your delivery -- but you will want pretty detailed notes and keywords on which to ground your extemporizing. Almost every sentence you plan to say should be scripted into a prompt of some kind, perhaps a brief point-form note including some keywords for you to use. Don't assume that, nervous as you will be, you will be able to expand a single, cryptically written point into a clearly delivered five-minute segment of your seminar.
How will you ever fill all that time? In fact, filling time is not so often the problem in seminars; running over-time is! You will need less material than you think. Don't forget that your oral delivery will have to be slower and clearer than your normal street patter. Getting through your points, drawing attention to and explaining your visual aids, dealing with spontaneous questions -- all these things take longer than you imagine they are going to take.
Hence the need to limit your topic (if you have a say in the matter) and to plan the flow of your seminar carefully. Have your detailed notes and your visual aids prepared at least a day or two ahead of time. Then practice delivering your seminar out loud (yes, out loud, not just in your mind!). If you have a friend (and don't mind straining the relationship), practise on your friend.
Among the valuable feedback you can get from your friend (aside from the certain news that you are talking too fast -- slow down!) is a check on whether or not your points are immediately clear. Again, the seminar format does not easily allow for backtracking. You may have to explain some ideas in what seems to you an overly primitive way in order to get them across with immediate clarity.
What should your overall plan be for maximum clarity? A usually good plan is to move from the general to the particular.
First, announce clearly what the topic of your seminar is; don't assume your classmates already know!
Then state what your central idea or thesis is.
Then make your main subsidiary points.
Then, by way of expanding on or exemplifying these points, explain the details of your topic.
Finally, sum up your presentation, to make sure your classmates have got the main points you've wanted them to get.
Remember: they should know at all times where you are in your progress through the topic and where you are heading. Only clear and detailed organization will keep them with you.