If you see a guild that is doing something and could use a little bit of help to do things more efficiently, you might consider asking them if they could use some help. Later on, that guild might help you. Just don't demand that they help you. That would be bad.

The guilds you should be most concerned with helping are allies and those you wish to ally with. You can consider helping other guilds too if you are feeling generous.

The best things to help with are those things that won't be much of an inconvenience. Bag sitting, death bag retrieval, harvesting a bag and moving a bag are some things that could considered. If the task would be too much of an inconvenience, it might not be a good idea to do it because it could make you annoyed with that guild and make relations difficult.