Hello!
Welcome to my first guide.
I'm not very experienced with guides so don't except so good guides from me ^_^
I'm doing my best to make this a good guide!

Marks:
Greater Mark of Alacrity
+1.7 Attack Speed
Or
Greater Mark of Insight
0.95+ Magic Penetration

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Early attack speed is really good for Teemo
Since you are going shredder you need some magic penetration, I don't always get these but I do sometimes.

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Seals:
Greater Seal of Resilence
+1.41 Armor
or
Greater Seal of Alacrity
+0.76% Attack Speed

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Because you are Teemo you often get focused and you are really squishy, a good choice for teemo.
More attack speed = more damage, but we are not going glass cannon so I personally don't recommend this so much, but its your own choice.

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Glyphs:
Greater Glyph of Warding
+1.34 Magic Resist
Or
Greater Seal of Alacrity
+0.64% Attack Speed
Magic Resist is good for Mid Teemos.
Again attack speed = more damage for shedder Teemo, I do recommend the Magic Resist glyphs more.

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Quintessences
Greater Quintessence of Potency
+4.95 Ability Power
Or
Greater Quintessence of Swiftness
+1.5% Movement Speed
AP Really gives you a good bonus into your Poison Darts which deals more damage.
Teemo is a very mobile champion because of his Move Quick ability, But sometimes I feel abit slow, So I use these if I'm playing solo top Teemo.

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I go 21 in Offense 9 in Defense and 0 in Utility
Why?

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Offense
In the offense tree, The extra AP from Mental Force boosts up your early damage on Poison Darts damage, The cooldown reduction also helps with mushrooms around the map.
Arcane Knowledge
+10% Magic Penetration
This mastery gives you magic penetration over tanks that build magic resist, if you get the Greater Mark of Insight, this really helps Mid teemo.
Blast
+1 AP per level (+18 at level 18)
More ap more damage from your Poison!
Archmage
Increases ability power by 5%
More AP :)
Executioner
This mastery is good for
-People who run into Mushrooms after team fights with low health and they hit your mushroom and die
-Good for 1v1 Fights.

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Defense
Resistance:
+2 Magic Resist
Magic resist = more sustain for the little squishy Teemo
Hardiness
+6 Armor
A good mastery if you are up against 2 ads.
Durability
More health = more sustain.
Veterans Scars
+30 HP
really really useful! For Teemo

Against 2 AD
Max out Blinding Dart first and blind the one thats doing the most damage.
Next up is Poison dart
Max up Move Quick last
Against 1 AD and 1 AP
Focus on Blinding Dart if the AD is really annoying, If the AP Has alot of skillshots max out Move Quick to dodge them, If you really want to harass max out Poison Dart.
Against 2 Tanks
Max out your Poison dart
Then max out your Blinding Dart
Then its time for Move quick
(Max move quick if you are against a Mundo or Shen to dodge his taunt/cleaver.

Thank you for reading my guide, I know it is not the best guide ever
And not even that good but Its my first guide
I hope you liked it and Enjoy!