what shall we do?

Acts 2

v38

Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost.

Is this verse the gospel message? No, and it's not all that Peter said at the time. Is it what we must do in order to have eternal life? No, these are all great things to do, but we don't need to speak in tongues to be saved, we don't need to be immersed in water to be saved, and we don't need to repent in our actions in order to be saved.

Let's look at this verse in context. Peter had already quoted earlier, in the same chapter,

What We Need to "Do" to be Saved

v21

And it shall come to pass, [that] whosoever shall call on the name of the Lord shall be saved.

This, of course, is talking about calling on the name of the Lord out of faith, not like saying magic words. And Peter went on to tell the gospel, of who Christ is and what He did for us on the cross, and in light of His resurrection.

Who He Is

v22

Ye men of Israel, hear these words; Jesus of Nazareth, a man approved of God among you by miracles and wonders and signs, which God did by him in the midst of you, as ye yourselves also know:

v36

Therefore let all the house of Israel know assuredly, that God hath made that same Jesus, whom ye have crucified, both Lord and Christ.

His Death on the Cross

v23

Him, being delivered by the determinate counsel and foreknowledge of God, ye have taken, and by wicked hands have crucified and slain:

His Resurrection

v24

Whom God hath raised up, having loosed the pains of death: because it was not possible that he should be holden of it.

This is the good news, the gospel, according to the Word of God, the Bible, and this is the message we're saved by believing.

v37

Now when they heard [this], they were pricked in their heart, and said unto Peter and to the rest of the apostles, Men [and] brethren, what shall we do?

The key reaction here, was in their hearts; that's where we need to repent, by having faith. What they asked next, was about actions they could take, what shall we do? That is why Peter answered with telling them things to do, like being immersed in water; not because some action was needed on their part for them to be saved.