Based on your zip code, it looks like you're near Vallejo. I'm in Sacramento. We're really lucky here that our climate lets us grow just about any tree outside year round, with only a very few exceptions (namely, tropicals). Depending on how close you are to the coast, you might even be able to keep a ficus outdoors in the winter.
Since you're new to plants, I'd suggest that you resist the urge to buy expensive, complete or near-complete bonsai plants now. First, learn to keep cheap trees alive, then move on to more expensive ones. With that said, your local Home Depot, Lowes, or other nursery can be a great place to find trees like this. I'd suggest that you begin with some trees that are easy to care for and are fast growers. I'm sure others can add to this list, but some starters could include: Boxwood, pomegranate, cotoneaster (there are a few different kinds), privet, juniper, and Chinese elm. Japanese maples can be a little fussy, so those might not be the best beginner trees. Trident maples are less fussy, provided that you give them a little afternoon shade. Junipers would work great. Shimpaku and procumbens nana junipers are very often used for bonsai, though just about any that are sold in a nursery around here will work fine.
If you want to visit a local bonsai nursery, Lone Pine Nursery in Sebastopol is awesome. If you ever make it to Sacramento, Maruyama Bonsai Nursery is very nice too. The elderly Japanese couple who own it are very knowledgable and helpful.
