Hello and a warm welcome to you, Koneko-chan. I hope you enjoy it here. ^_^
You can upload an avatar by going to your User Control Panel (upper left), and under the top "Profile" tab, you can select "edit avatar". Currently, you can only upload an avatar from your computer.
As far as looking for one goes, that's a bit more difficult. There are many places to find an avatar (I usually skim through livejournal posts with icons, though I'll also just take an image I like and edit it to avatar size). I'm sure there are others who can give you good suggestions, though. ^_^