Though some have proposed to give "simple" answers to this question (see here) explainations are often very vague and confusing. For example, they will get repentance and faith mixed up, they will make it unclear exactly WHEN you are saved, etc.
We know that something as fundamental as Salvation cannot be so mysterious and ambiguous - So here it is:
For the unbeliever: Acts 16:29 - 32 Then he called for a light, ran in, and fell down trembling before Paul and Silas. 30And he brought them out and said, “Sirs, what must I do to be saved?” 31So they said, “Believe on the Lord Jesus Christ, and you will be saved, you and your household.” 32Then they spoke the word of the Lord to him and to all who were in his house.
Do you already believe but have not yet repented?: Acts 2:36 - 38 “Therefore let all the house of Israel know assuredly that God has made this Jesus, whom you crucified, both Lord and Christ.” 37Now when they heard this, they were cut to the heart, and said to Peter and the rest of the apostles, “Men and brethren, what shall we do?” 38Then Peter said to them, “Repent, and let every one of you be baptized in the name of Jesus Christ for the remission of sins; and you shall receive the gift of the Holy Spirit.
Do you believe and want to do right, but have not yet told anyone (confessed)?: Acts 8:36 - 37 Now as they went down the road, they came to some water. And the eunuch said, “See, here is water. What hinders me from being baptized?” 37Then Philip said, “If you believe with all your heart, you may.” And he answered and said, “I believe that Jesus Christ is the Son of God.”
Are you a confessing believer who wants your sins washed away?: Acts 9:6 So he, trembling and astonished, said, “Lord, what do You want me to do?” Then the Lord said to him, “Arise and go into the city, and you will be told what you must do.” Acts 22:12 - 16 “Then a certain Ananias, a devout man according to the law, having a good testimony with all the Jews who dwelt there, 13came to me; and he stood and said to me, ‘Brother Saul, receive your sight.’ And at that same hour I looked up at him. 14Then he said, ‘The God of our fathers has chosen you that you should know His will, and see the Just One, and hear the voice of His mouth. 15For you will be His witness to all men of what you have seen and heard. 16And now why are you waiting? Arise and be baptized, and wash away your sins, calling on the name of the Lord.’
To sum it all up: Mark 16:16 "He that believeth and is baptized shall be saved."