Either go through notifications and if anyone has helped you, you can help in return with the handshake symbol, if the handshake symbol is gold you will get supplies aswell as gold for helping them, or you go to the provinces map and enter people cities and use the hand symbol at the borttom of the screen to help there, if you are on a PC watch for a treasure chest drop. and finally if you are in a fellowship, use the fellowship button to help your fellows. also most people have at the top of there screen whether they want culture, main hall or build hall help, so try and follow that.
That is, assuming you are playing on desktop or laptop. If you are mobile go out to the map and click on each city. You can help or view their Fellowship/invite to yours if you are ambassador, mage or archmage (if they do not have one).
Your topic header says culture task, are you on a quest say to get 150% culture? if so you either need to build some culture buildings or get people to help your culture buildings. you helping people wont help raise your city culture, if thats what you are trying to do.
(but still help others so they return the favor)
For Neighborly Help, you can also provide to those on your world map (and hopefully they will return the favor). I have build good relations with a percentage of my neighbors this way. It can help out in a bind, for trades, etc.