I think this works but im not sure if anyone has other method plz tell me

I have at least 35% lifesteal and have Last Wisper with me

Armor pen, more health/survivability, and life steal.

Pick up magic resistance which will mitigate the effects of the Thornmail passive. Armor penetration helps you do more damage for the life steal.
But as a DPS, don't focus the tank. You shouldn't have this problem.

well this is what iv herd while playing online to counter a thorn mail the best bet is to get a warmog's armor

Basically mix in LW + any kind of survivability item and you'll be fine. Warmogs will let you troll normal games, in ranked, LW+BV with some Lifesteal is enough.

Get magic resistance and save the tank for last.

If the tank is building Thornmail and he doesn't have a taunt, that tank is doing it wrong.

Tanks without taunt have enough trouble giving enemies a reason to attack them... adding more reason not to really isn't necessary.

Warmogs is always the better choice. Do the math on total mitigation done over the 45 seconds from tank with thornmail on at a level you feel is good for someone to have gotten thornmail or u can farm a warmogs.

Usually depends on enemy team comp for best choice.
High magic: get FoN or BVeil
High Physical, low CC: Go with Last Whisper and more life steal.
High Physical, high CC: More health and health regen. It doesn't help against thornmail but you'll survive longer.
Mixed Physical and Magical team, low CC: Revive armor and life steal
Mixed Physical and Magical team, high CC: BVeil and Revive armor. two items to give you defense, mitigate the magic damage from thornmail, and help you survive.

As always try and pick up an item that helps your dps but also gives you some added health so you can take more abuse while upping your dps.

If they don't have much magic resist then you can go Bloodrazor combined with armor pen and possibly lifesteal.