^ the items are pretty easy to figure out after awhile.
the most important thing u want to do is buy items that will help you survive the first 7 levels with your starting gold. basically don't buy anything over 400g when u start. ring of regen, sobimask, ring of the basilisk, tangos are all good choices because they keep you in your lane awhile.
most people try to go straight for super good items and that's a bad idea. the best items u can get (for a public game atleast) are a boot which you can upgrade to a powertread (few characters need phase boots or boots of travels so don't even think about those) but only upgrade it after you've completed 3 nulls, wraiths, or bracers depending on your hero. these 3 items will make you WAY more powerful and cost almost nothing. 5th item is some kind of item to increase your survivability (Hood, vanguard, linken) and 6th item is your end game item (butterfly on most agilities, radiance for some other heros, etc).
there's some deviation in there but you can't go wrong with that strategy to begin with.