Introduction to Early Game Items
Having a solid early game and staying in the lane as long as possible is the most important you need to worry about.
First item is Runes of the Blight. If you aren't picking these up, you're probably doing something wrong and you should re-evaluate your plan and start over with buying a set of these. Health potions are not an acceptable replacement for the runes for the reason that they can be dispelled. This doesn't mean you shouldn't pick up a potion at all, it just means if you are going to pick up a potion, you should already have some runes in your bag already.
Health Potions will give you 400 hp while runes will only give you 345. The thing to remember is most heroes have to get way too low to need the health potion early on, in addition to the fact that it is dispelled on attack. Best to have one of these for a bit later, just to stay in the lane for a bit longer.
Mana Potions are an item that you really want to pick up one of on some of the early int characters to keep them effective early on. Toss yourself an extra 100 mana for another early nuke on the enemy carry and you could ensure a victory for your team, if it's done just right.
Ward of Sight and Monkey Courier if you are a support, better yet, if you are not a carry and your team does not have a set of wards and a monkey, cough up the cash. Buy the monkey, buy the ward, your team needs them and if the enemy has them and you don't, you've probably lost the early game. These two items are more important than that crown you wanted, or the mana battery. You picked a non-carry, so support the team. As a general note, don't buy both unless you want to get screwed really hard early game.
Bottle (3/3) is an early game item that you should only pick up first of someone like Thunderbringer, if you aren't warding. Bottle should be reserved for after you pick up 600g in the laneing phase and you run it to you from the team monkey. You need the stats from the next few items to last hit better against opponents who aren't going to go bottle. TLDR: Last hit power > bottle 90% of the time.
Minor Totem/Pretender's Crown make for a nice HP, mana, and last hit boost that you really can't afford to leave behind. generally you want to pick up 2 Minor Totems because stat point for gold on these are through the roof in comparison to other items. Pick up the crown only if you are saving space in your inventory.
Mana Battery can be bought in the outpost, but sometimes you want to pick this item up before you get there, such as when you have a heavy spamming lane or you are going to be roaming. This is not a buy for everyone at the start, but it is an item that makes the game more interesting, faster paced, and much more entertaining. Mana battery great, but be cautious about turning it into a power supply, because the recipe cost can usually be spent better on other things. If you need the inventory space and you are getting a lot of use from your battery though, a supply is a valid option.
Logger's Hatchet is really only an early game item for melee carry heroes. You won't pick it up often and you will have to give up stats to pick it up right away, but it will allow you really great last hits. Know who you will probably be laneing against and decide if the lowered health and mana is worth the better last hit.