I assume you have not done any aikido before? I'm not sure what style of aikido you have joined up with, but no doubt you would have at least checked out a class before doing so?
In any case, be prepared to fully empty your cup. Aikido is unlike any other martial art. You have to let go of any preconceived notions of being able to physically "do" a technique. The keywords you will most likely constantly hear are: relax, soft, extend (ki), move, flow, and feeling.
You may or may not experience people simply falling over for you. Do not put too much weight in that, as to aikido's effectiveness with a (too) cooperative partner - it will come. It will take at least 2-3 years before it comes. It will take at least another 4-5 years to fully let go.
The key points of aikido is to practice your posture, your footwork and your mind - treat it as "moving meditation" - and let the techniques happen naturally.
It is easy to get frustrated with aikido initially, but if you can stick with it for at least a year, you will be hooked.
Most of all, work harder at your ukemi than on the throwing. The secret of aikido is in the ukemi. Learn how to do it, and do it well. When your ukemi is of a high enough standard, your throwing techniques will be exponentially more powerful than the person who doesn't like doing ukemi.
Above all else, have fun! And as the founder once said, "Train in a joyful, vibrant manner".