if you're trying to play with a friend, you'll need the engraved ring, bought from the cat in majula. use the same god and place the summon sign with the ring on. after you two connect, both of you can take the ring off. co-op is determined by soul memory, and has different tiers. refer to this page: http://darksouls2.wiki.fextralife.com/Soul+Memory

What game?

In Dark Souls I, I believe the only factor on who could/couldn't play together (as far as stats) was your Soul Level. In Dark Souls II, you also had to take into acvount your Soul Memory (how many Souls you got total) as well as your SL. In 3, I think they went back to 1's system. Other than that, make sure the host (whoever's summoning) is in an "active" state (Human in DS1/DS2, Embered in DS3), by consuming a Humanity (DS1), Human Effigy (DS2), or Ember (DS3), and make sure the host has not beaten the boss of the area. In Dark Souls II, using the small sign soapstone extends the range of soul level compatibility, and lets you play in areas where the boss is defeated. Using the god name ring extends it further, but does not give the other benefit. (they can be stacked) In Dark Souls III using a Password almost guarantees a connect, though a higher level person is weakened to match the host. I feel there's one more thing I'm forgetting.. I'll edit it in should I remember.

You have to use humanity (DS1), human effigy (DS2), or embers (DS3) to be able to use summon signs. Once used, you will be able to summon any player from their summon sign. Most white summon signs for help are placed outside a boss room. Be warned, however: should you die before summoning a player, you will need to use another humanity item to be able to summon again. However, you can use your white soap stone to be summoned at any time.