The item you seek is the "white soapstone" that allows you to place a white soul sign down for others to summon you if they are in their skin. It can be earned by talking to the knight staring at the sun after the Taurus demon mini-boss.

I looking for help on dark soul

If you're asking how to directly jump into a friends world... you can't. Trust me if you could, it'd break the game. The best way to try is to get the White Sign Soapstone from Solaire of Astora who is a knight out on the battlements towards the sun when you have your back to the big ugly red dragon of the bridge. Using this will make a sign appear in other peoples worlds which lets you come there and help them.

What will help is if you join one of 3 covenants. The way of the white, Princess's Guard or the Warrior of Sunlight covenants. When in these covenants (and are human), summon signs that appear are more likely to be of members from these covenants. EG: If you and your friend both join the Princess's Guard Covenant, then go to an area where you haven't defeated the boss AND are both human then there is a greater chance that you will be able to see the summon sign of each other

Note: The Princess Guard covenant requires you to have gotten the Lordvessel