So many questions, young padawan!
Regarding Wizards and Staves: You can find magical staves, but they're pretty hard to get a hold of, especially at level 3. And when you do, they rarely make you a better fighter or anything like that. Nope, they usually just let you toss out fireballs or huge, world-destroying spells. Nothing interesting or anything.
Regarding scrawny wizards: Yup, that's a thing. Armour isn't going to affect your hit points, and to get proficient in, say, medium armour is going to cost you either multi-classing, or two feats. I'm guessing you're not using those rules yet. In which case, no, you can't. But don't worry - it's a bad move anyways! Look at the spell "mage armour", cast it frequently, and hide in the back whenever you can. You shouldn't be getting hit anyways. Hit points are for people who make mistakes. Don't make mistakes, and you're golden.
11 hit points at 3rd level, while not the greatest, isn't terrible. At first level, you'll have 6 hit points, plus whatever your constitution modifier is. Each level, you roll 1d6 plus that modifier. Assuming you have no constitution modifier, 11 at 3rd level is about right. If you do have a con mod, it's a little low, but oh well. Put the fighter between you and the bad guys. That's his job. You went to sage school! You shouldn't be getting hit! Outsource that job to someone with armour!
After first level, sage means nothing to your character advancement. You're safe. At this point, the only thing it's going to lead to is book time and using your sage feature to find more quests.
Anyways. That answers your questions. Now, the most important answer to something you didn't ask:
don't worry about any of it. Seriously. Don't stress the rules. This isn't a board game, where if you're not following the rules, it's "Cheating" and maybe giving you an unfair advantage. If you and your GM can agree on a way to resolve something, and it's not the way it's done in the books, that's absolutely good to play with. You'll hodge podge together your own rules, play for a while, have the game collapse due to everyone screwing up, re-read the rule book once you sort of know what you're doing, and start again.... with lots of memories of that first campaign.
Seriously, it's been more than twenty years since I ran my first "real" campaign, and it still gets spoken about. And we used about 15% of the rules in the damned book.