You should get boots if you're against people with great movespeed and cc themselves. Elsewise start the damage cause it's better to outdamage them than run from them in most cases. Running exposes you to a lot of damage and you want to avoid that early game. Also consider if they have high sustain (Soraka or Sona)

Assess whether you're doing well or poor first and judge whether damage or safety would help you. Phage makes you much tankier than your initial stats, and it adds a slow to your auto attacks, which enables kiting as well as deadly chasing. very good. But malady adds a lot more damage per hit and over time. If they're squishy it can be beneficial to go the damage route, but sometimes it's still better to go with phage even if they are squishy. o-o If you're having a lot of trouble get boots first because Teemo has a naturally high mobility and can escape things rather than try to outright tank and live.

Okay. Here's what the items do for you so you can decide what you want on a Per-Game-Basis.

Liandry's Torment - Adds a lot of magic penetration + some flat ap which will make you BURST HARDER, as well as Liandry's special passive makes your mushrooms do upto 12% of their health ONTOP of the mushrooms damage. That depends on how much magic resist penetration you will have.

Frozen Mallet - Makes you much tankier and enables you to always slow people, as well as adding a tiny bit of damage to your auto attacks (Not much at all)

Rylai's Crystal Scepter - This is like the AP version of Frozen Mallet, it will make your Q do most of the slowing as well as make your mushrooms slow EVEN more. This will not enable attack kiting, it will enable BURST kiting. Q, attack(lichbane proc), run away if you can't kill them with auto attacks.

Wit's End - This is not a very good item in the midst of a squishy damage build. Only build this if you're getting Frozen Mallet too. This item gives you some resist while adding a pretty big chunk of damage to your auto attacks. As well as big attack speed bonus. On hit build ONLY.

Morellonomicon - This will sustain your mana pool and give you a high CDR, which means more mushrooms and blinds faster. In turn this has the potential to raise your BURST damage a tonne, but does little for your attacks unless you have malady, then it does a fair bit (More than Frozen Mallet atleast)

Nashor's Tooth - This will boost your attack speed and CDR significantly, plus some AP. This would be a better choice than Morellonomicon in most situations, it's just that with the added CDR your going to start becoming mana deprived. If you take blue buffs or back to base frequently doesn't matter.

Lich Bane - This is a huge damage addition and provides movespeed as well as making you capable of burst damage in combat. Core item, should almost always build.

Rabadon's Deathcap - This is the thing that will make your AP go from 200 to 500, it adds the most to your damage once you have adequate magic pen. Core item, should almost always build.

Void Staff - This let's you penetrate everyone's MR no matter how much they have. Core item, should /always/ build.

Malady - This adds a bit of AP and makes your attacks do a tonne of damage as well as shred MR. Core item, should always build but it's getting removed next patch. Replace with Wits End next patch if I forget to update build.

If you're going for burst damage go

Malady -> Rabadons -> Void Staff -> Lichbane -> Morello, Nashor's, or Rylai's.

If you're going for onhit go

Malady -> Wits End -> Void Staff -> Frozen Mallet -> Nashor's

Build boots whenever you want before your second major item.