Though Baptism by Water is not necessary to go to heaven, it is however very important to your relationship with God.
Ephesians 2 (NASB)
8 For by grace you have been saved through faith; and that not of yourselves, it is the gift of God; 9 not as a result of works, so that no one may boast.
Romans 10:9-10 (NASB)
9 that if you confess with your mouth Jesus as Lord, and believe in your heart that God raised Him from the dead, you will be saved; 10 for with the heart a person believes, resulting in righteousness, and with the mouth he confesses, resulting in salvation.
Neither one of these says you need to be baptized by water to go to heaven. However, some like to point out Mark says baptism is essential to salvation.
Mark 16:16 (NASB)
16 He who has believed and has been baptized shall be saved; but he who has disbelieved shall be condemned.
When looking into it, there is a lot of evidence that this section of the book of Mark is not supposed to be in there. Our oldest manuscripts do not have the last section of Mark, which means it very well could have been added to later on by one of the scribes who copied the original manuscripts. There are cases of this throughout the Bible where one word or part of a verse was added or removed by the scribes who were copying the manuscripts. Another reason they believe this was added later is because inside the last 11 verses of Mark, there are 17 new words that were not used even once through the rest of the book. These words are all considered to be non-marcan, which means they are not characteristic to the usage of Mark's writings.
Even if one does believe this verse belongs, there is one significant point I would have to make. It never says what kind of baptism it is talking about. Everyone who accepts Jesus as their Lord and Savior is baptized by the Holy Spirit the moment they have that faith and confess aloud that Jesus is Lord. Mark could easily be clarifying that he who believes must believe in the heart where they are baptized by the Holy Spirit and not have a dead faith like James mentions.
So the only place in the Bible that mentions baptism in connection with being saved is very questionable to even be authentic to the original manuscripts and even if it is, there is no evidence it is speaking of Water Baptism. Everywhere else, our salvation is based on our faith and nothing more.
So why do we get baptized?
Though it is not required to go to heaven, it is a commandment from the Lord. The reason I believe is because it is essential to our relationship with God.
Matthew 28:19 (NASB)
19 Go therefore and make disciples of all the nations, baptizing them in the name of the Father and the Son and the Holy Spirit,
1 Peter 3:21 (NASB)
21 Corresponding to that, baptism now saves you—not the removal of dirt from the flesh, but an appeal to God for a good conscience—through the resurrection of Jesus Christ,
Acts 22:16 (NASB)
16 Now why do you delay? Get up and be baptized, and wash away your sins, calling on His name.’
Baptism by water is an act of dedication, symbolizing the washing away of the old self and the resurrection of your new self. It is a dedication of your love and desire to be right with God. Jesus was baptized. Jesus told us to be baptized. For that reason, to choose not to be baptized is an act of rebellion against what Jesus commanded.
Let's be serious here. After all there is nothing more serious than your salvation. You are saved the moment you fully believe with all of your heart that God raised Jesus from the dead and confess with your mouth that Jesus is Lord. At that moment, you are entering into a relationship with God and placing a target on your forehead for the devil. How well you stand against the devil's attacks will be dependent on your relationship with God. Push God off to the side and continue living in the world and Satan will have a field day with you. You don't want that. You want to immediately start building on your relationship with God. How do you do that? Just like you do with any other relationship in life.
The keys to a good relationship are simple.
1) Getting to know God better.
How do you get to know God better? Reading and studying His Word. Not just reading it, but studying it as well. The better you know the Bible, the better you will know God. You should read it every day, at least one chapter. Find time to spend studying it as often as you can.
2) Love God.
How do you show God love? Two ways. One is through praise and worship. The other is by doing His will. How do you do that? Jesus made it clear. Love your neighbor as you love yourself. When you show others love, you are loving Jesus.
How do you communicate with God? Praying! Pray when you wake up. Pray before you read His Word. Pray throughout the day. Pray again wen you go to bed.
How do you forgive God? You don't because you never have to. God will never need your forgiveness. He needs you to ask Him for forgiveness. Like any relationship, if you don't forgive the person for things they do that hurt you, the relationship will not last. That unforgiveness will build up into a wall until you are cut off from each other. Every time you pray, start the prayer off by asking God to forgive you for any sins you have or may have committed.
Look, any relationship takes commitment and there is no relationship more important than your relationship with God. And when you are first saved, you make that commitment clear by getting baptized as soon as you can. There is nothing that would scream that you are not committed more than putting water baptism off for no good reason.