im new to unity and i want to know how can i make characters and create/download other textures
Also, take a look of the links suggested in the replies to this question: Where can I find art/models/assets for my game?
If you make a simple game and you don't need specific characters you can find generic ones (like female, male, alien, ogre, orc, etc.) for free on the website from those links.
Well to make characters you need a 3D modeling program, and for that might i suggest Blender3D(free)? And for textures just search google, there are a lot of good sites, or you can get Photoshop or GMIP(also free) and try to make textures.
Hope I Helped
If you're on a mac, Cheetah3D is an affordable 3D application that's quite powerful and a lot easier to use (or at least getting into) than Blender. There are many 3D asset websites that have many free items, like turbosquid. Don't expect too much of the models as many aren't really build for gameengines and have way to many poly's or just look crap. Good for getting started though.
You can try http://www.texturewarrior.com You can download FREE 3D texture maps and materials