If your commute is not too long (under 8 miles each way or so) & you already have a decent backpack, I'd use that to start until you settle into what you will need to bring. I use a Mountain Hardwear Scrambler myself & it's just about perfect for me (A little bit large, but it works great if I have to stop & pick up groceries or anything on the way home). I second TJ's recommendation for leaving stuff at work. If there is any way you can, leaving stuff there will make your life way easier. Shoes especially, they are the hardest thing (at least for me) to pack. Aside from that, toiletries & a towel would be great to leave at work, along with a spare outfit (I leave socks, underwear, shirt & Pants).