In the astral plane, I heard theyre evil entities that can try to possess and Im trying to figure how to avoid astral demons. Is there a way that an evil spirit wont be able to take control over my body?
Demons are not inherently bad. No spirit is inherently "evil" nor out to get you. Any spirit can manipulate, abuse, or trick a person. As stated before, true possessions are very rare and almost never even heard of. You do not need to avoid demons, I have worked with a lot of friendly demonic spirits, as well as some troublemakers, they are not out to get you, nor will they attempt to enter your body.
Energy shielding, warding, and protection is a must before, during, and after any sort of spiritual interaction.
What you have heard about demons only being bad is fear mongering. Not the truth.