It's entirely based on the spirits and deities you already have a relationship with.
When making a petition or oath, you are sacrificing something to them in a way. Sometimes, this could be a vice such as excessive drinking. This could be an oath to not take your own life. You could solely worship that spirit, and keep your practice only between you and them. Or, an agreement to quit smoking. It's an agreement made between the two of you. These actions have meaning to the individual and the spirit. When there's a relationship there before the oath, it has more meaning. You'd be more inclined to not break the oath. Would you keep an oath more to a stranger or to family? Would an oath made to you by a stranger mean more than one from a close friend?
In Hellenic Polytheism/Hellenismos, the point is to build kharis, or grace. It refers to favor, a relationship between mortals and gods. This virtue sort of spreads to a lot of spirit working practices. Essentially, it helps develop a stronger sense of unity between that person and their spirits. Any of those things listed about can be done without an oath. The Delphic Maxims say to avoid making an oath. You can make a guess as to why.
Petitions/prayers are a bit different. While it would be preferred to make one to someone you have a relationship with, it doesn't have to be. Usually the petition follows this sort of formula: "Hey, I give you an offering of your favorite things, and make you a nice space. I ask you give me __________. In return, I offer ______________." Of course, this should be better worded based on the spirit/deity.
Christian Saints and Prophets, Gods from different pantheons, spirits from differing cultures, blah blah blah can be petitioned. But, unless you know about them, what they like, don't like, what others traditionally petition them for, their spheres of influence, etc.
Heres the kicker: The spirit or deity doesn't owe you anything. Even if there is a relationship. They can decide to accept the petition or not, and it's their right. This is why divination is a good thing to know before doing any kind of spirit work.
You'll have to just use your own intuition there. What calls to you? Who do you love?