League of Legends (LoL) Question: How do i counter Morgana?

If you are an ADC and need to counter her, use Quicksilver Sash. You can build it into Mercurial Scimitar whenever you need to which will give you damage, lifsteal, magic resist, and a 90 second active to delete her rooting abilities. Adaptive Helm is another great item because it reduces the damage Tormented Shadow does.

you can counter that stun by RENGAR when you press (w) he wil disable the stun and anythings that affect him.

Morgana's passive lets her heal up by damaging enemies with spells, keep this in mind as each time your hit with an spell you dont only take damage but you also let her heal herself.

Her Q is an root, meaning you can still use spells and autoattacks while your bound in place.
Another thing about it is that you can dogde it by using minions as a shield.
And it travels in a streight line, which means you can walk side to side to make it a little harder for you to be hit with the Q. (obviously if you walk at her in a streight line its easy for her to hit her Q)

Tormented Soil (her W) has an passive CDR as the cooldown from it gets reduced by 5% each time her passive heals her, whuch is something to kinda keep an eye on.
Its annoying to be zoned out by this spell but to me its worth it to let it happen unless she already used her sheild and you know you can get in for the kill.

I dont have to much experience when it comes to how her shield works and figuring out what it will take to break it.
But when I played against her as Janna and I had a couple of points into my W I could pretty easily get rid of morgana's shield by using my W and an autoattack.
What I mean to say with this is that the shield seems weaker then it is and I would not always back off when you see her use the shield, the fact that disabling effects dont work is pretty annoying at times though.

Her R is just an matter of getting out of range, use flash or whatever you can and avoid the 3 second mark cause once you have been in her ult for 3 seconds you take extra damage on top of a stun.

It's mainly practice to dodge skill shots but these things should help:
1) Buy boots of movement items, as it easier to dodge that way (you can buy them earlier against skill shot champions).
2) keep distance as it take more time for the skill shot to get to you and you have more time to move.
3) If you just can't dodge it, buy Mercury's Treads or Banshee's Veil like item. and you may consider the tenacity rune Legend: Tenacity.

but mostly practice :D