how to position yourself should come intuitively over time.
what you should make an effort to learn is items, and in particular
why you get certain items, and skill builds, and why you pick skills in certain orders.
there are some general guidelines like "max nukeskills first", but those doesn't always apply.
here are some golden rules I follow. I'll start with mid tips:
don't start with bottle for mid, not even with AR money. spend money on stats, axe, or whatever helps you last hit, and tango/hppot to keep you alive until you can afford the bottle. going mid/bottle, I'd say zeus is the only hero who should still get manapots too, everybody else should just need hp reg stuff until bottle.
[I've started using this a lot recently] get wards for mid if you play a good harasser/blinker like storm, zeus, qop or whatever. place it on enemy mid ledge just outside range of tower. 150g is worth it, + you can ward 2nd rune (or 1st rune if your team is being gay) when getting rune. very nice playing as batrider too. helps vs pudge too.
playing as bloodseeker mid, you only need 1 hppot, or 1 set of tangos. the "heal on kill" works on denies too -- it's super easy to sit mid versus most heroes by just being a good last-hitter.
always get at few branches on juggernaut (magic wand is good on him). it's crucial if you want to ulti/ww combo early - and you'll want to.
highest damage item components are usually the most cost efficient, so if you're farming battlefury or something, try get claymore (1400) before broadsword (1200). obviously depends on stuff like a) some fag using courier or not, b) about to die.
drag items you are buying next into the Quick buy area in the bottom right. you'll hear a "ca-ching" sound when you can afford stuff. also, it makes it easier for you to quickly buy shit if you know you are dying. buying stuff before you die is something everybody should learn so that they don't lose unnecessary amounts of gold when they die. it's a little like starcraft 2 (but for different reasons): try to keep your gold as low as possible. exception to that rule: you are a nuker (being able to buyback and tp/get a kill can be worth it), or it's late-game (buy-backs can often be more important than items).
if your hero has skills that increases damage (razor's link, shadow fiend's necro), you should focus on getting hp/ultility or orb effect before going pure damage items.
when going for helm of dom/satanic, remember to charm a creep and stack ancients or a hard camp. also buy the armor/hpregen helm first, not the life leech thingy. pretty much all camps are: make creeps start moving away at 52-53, and it will stack.
tranquil boots are good boots for any hero going for vlad (you can break down into components). otherwise there's usually always a better alternative. it's good for ursa/phantom lancer, for instance.
pretty good basic builds for all heroes:
www.dota2alttab.com
edit: god what an ugly post. oh well it's 4 am, good night.