Hello suesue2002! I am new to the DISboards too! I have gone to Disney several times, always in March. While the weather was mostly in the 70s, it did start off cool in the mornings. I am by nature a cold person and I found it comfortable enough to wear shorts and just bring a hoodie or two. That way, I can switch out my shirts but wear the same hoodie and have an extra packed just in case I needed to change it at some point. When it got warmer, I took my hoodie and then wrapped it around my waist and I was fine for the rest of the day. I did bring one pair of pants just in case, though. If you wanted to wear pants and sweatshirt and then change when it got warm, there is also the option of renting a locker for $5 a day (it actually costs $7 but when you bring your key back they return $2).
So basically, I packed mostly t-shirts and shorts with a hoodie and a pair of pants. (Do not forget a poncho!) I was able to get everything into a small suitcase. Though you will be walking outside, a lot of the attractions and queues are indoors. I hope this helps!