Footwear in the dojo is usually optional. You take off your shoes just as you come in the door at most dojos, then some people like to have a pair of sandals that they wear when off of the mat, and others (like me) just tramp around barefoot. There's almost always a place to change, and it is generally not a great thing to wear your training clothes outside anyways as it brings dirt into the dojo. You probably won't be training long before you are wearing a gi anyhow. The sweat pants are a must to keep you from getting raw spots on your knees.
Some things to expect first off.
1. You probably won't have a clue what everybody is doing for the first couple of classes, just follow along as best you can with the bowing in and such. This is especially true if the new class doesn't all start at the same time so you would be training with people ranging from zero to three months of experience.
2. For most people, seiza is something that you get used to over time. It really does get more comfortable after a while.
3. When in doubt, bow. Works for lots of situations
Hope you have a great time starting your aikido training!
Best of luck,