Du'a (supplication/requesting/calling) is divided into two types:

THE FIRST: دعاء العبادة - supplicating to Allah by performing acts of worship, such as: prayer, fasting, etc. Q. How is this a supplication? A. If a person prays or fasts, he is actually asking Allah -by means of this deed- to reward him and protect him from punishment. NOTE: Directing this type to other than Allāh is major Shirk without any exception. THE SECOND: دعاء المسألة - supplication to Allah by directly asking Him, eg. O Allah forgive me, save me from the Hellfire, etc. NOTE: Directing this type to other than Allah is NOT always Shirk, i.e. Asking the creation for something is not always Shirk. For example, asking someone for something they are capable of is not Shirk. Eg. Give me a glass of water. The Prophet ﷺ said: ((إذا دعاك فأجبه)) “If he calls/invites you, respond.” Similarly, Allah ﷻ said: ﴿وإذا حضر القسمة أولو القربى واليتامى والمساكين فارزقوهم منه﴾ “And when relatives, orphans, and the needy are present for the division, then provide them from it.” [An-Nisa: 8] This if the poor person extends his hand and says: Provide for me; this is not Shirk, because Allah Almighty said: ﴾فارزقوهم منه﴿ But if one requests from the creation something that only Allah can do, then this is Shirk. An example of this is, calling on a person to send down rain, believing that he is capable of doing so. Summarised from: "Al-Qawl al-Mufid ‘ala Kitab al-Tawhid" by Sheikh Ibn Uthaymeen rahimahullah