Romans Chapter 10. Verses 9 and 10 says:
If you declare with your mouth, “Jesus is Lord,” and believe in your heart that God raised him from the dead, you will be saved. For it is with your heart that you believe and are justified, and it is with your mouth that you profess your faith and are saved.
this is how I describe what is written in scripture above-
If someone says (in their heart or out-loud) ...
in essence- "Hey , I haven't been very good at being my own Lord- (running my own life) and I need help.... So yeah, I do believe God raised His only begotten Son Jesus from the dead and I'd like Him to be Lord of my life now.
at that very second soothing supernatural happens. it's the greatest miracle one will ever experience. becoming a child of God and and a brother or sister of Jesus Christ.
from that time on one has the legal right (legally recognized according to God and the evil one and every other being) to use the name of Jesus Christ to command evil spirits to go.
Using Jesus' name is like using like the power of attorney in a court of law. When one is granted power of attorney he is granted the full force and power that stands behind that name he is using.
and as an aside- in using the name of Jesus Christ - don't get caught up in the whole "I cast you demons into outer darkness" type stuff you may have seen in movies or heard some preacher say. That kind of bravado is unnecessary and can cause arrogance. check out this verse.
Jude 1:9
“Yet Michael the archangel, when contending with the devil he disputed about the body of Moses, durst not bring against him a railing accusation, but said, The Lord rebuke thee.”
this is referring to a stand off between the chief good angel and the devil. notice that an angel as powerful as Michael didn't start start bossing the devil around but simply said "The Lord rebuke you."
in other words I now call on God to back you down. the moral of this story is leave all the spiritual "battle" stuff to God and Christ. They can see all the entities we can not. they know all about hem. we do not. so we are instructed to simply use the name of Jesus Christ. here is one such instruction by Jesus Christ Himself-
Mark 16:17
“And these signs shall follow them that believe; In my name shall they cast out devils; they shall speak with new tongues;”