Teemo in the current season is not the best champion to pick. He is one of the weakest champions at the moment, but if he is played correctly, he can mess up his opponents.

This short guide is for low ELO players in Bronze, Silver and low Gold. Any higher than that, this guide might not be viable.
The guide is written from my own experience in the lower ELOs with Teemo and the builds is what I have tried and that is working in the current season and meta.

Teemo is a hybrid damage champion that scales the best with AP, but works just fine with AD as well. His basic attacks are AD based but his E (Toxic Shot) deals bonus Magic Damage. So AP is the best way to go right now.
AD is better if your opponents are building a lot of Magic Resist. So if your team has heavy AP damage, build AD Teemo with only Liandry's Torment as your AP item due to it's combination with Toxic Shot. That will give you higher basic attack damage over-all.

Teemo's passive is Guerrilla Warfare. After Teemo stands still for 2 seconds, he becomes invisible to all units. If Teemo walks inside a bush, he will become invisible after 2 seconds and can move around in the bush and still be invisible.
This can give a decent Teemo player a good advantage against ganks. If you are getting ganked, try to get into a bush and try to hide. Remember that as soon as you attack something you will lose your invisibility.

Teemo's Q is Blinding Dart. It's a click-and-shoot ability that deals magic damage and blinds the target for 1.5 seconds at level 1 and 2.5 seconds at level 5. The enemy will miss the basic attacks during that duration.

Teemo's W is Move Quickly. Teemo gains extra movement speed when activated.

Teemo's E is Toxic Shot. Each basic attack deals bonus magic damage and poisons the target, dealing magic damage over time.

Teemo's R is Noxious Trap. Teemo places a mushroom at target location. The mushroom becomes invisible after 1 second, also detonates after 1 second if opponent or enemy unit walks over it. Can not detonate on Baron Nashor. Teemo can store up to 3 mushrooms at the time and has a 1 second cooldown if you have multiple mushrooms stacked.
If a mushroom is placed on another mushroom, it bounces straight ahead.
If a unit or opponent get hit by a mushroom, they take magic damage and gets slowed for a short duration. Each mushroom lasts 5 minutes and gives vision in a short area.

First tree:
-Sorcery
--Arcane Comet (Damaging a champion with an ability hurls a damaging comet at their location)
--Manaflow Band (Hitting an enemy champion with an ability permanently increases your maximum mana by 25, up to 250 mana)
--Absolute Focus (While above 70% health, gain an adaptive bonus up to 24 Attack Damage or 40 AbilityPower (based on level). Grants 3 Attack Damage or 5 Ability Power at level 1)
--Scorch (Your first ability hit every 20 second burns champions)

Second tree:
-Inspiration (+12 Attack Damage or +20 Ability Power, Adaptive)
--Future's Market (You can enter debit to buy items)
--Cosmic Insight (+5% CDR, +5% Max CDR, +5% Summoner Spell CDR, +5% Item CDR)


These runes is the best at least for my type of play style with Teemo. You can change them but try to keep the Sorcery tree as close to the one I have as possible for the best results.

Teemo is not that great in team fights. His Blinding Dart can only attack one opponent at the time. If you end up in a team fight, stay future back and land Blinding Dart on their ADC or Top laner, depending on the fight. After that, spam mushrooms in the center of the fight to slow the opponents and deal damage over time.

Teemo is best at split pushing and harassing. Use that to your advantage. Your team WILL ping you to help and roam, but only do that if you really have to. Split pushing is the best option.