Baptize is from the Greek word "Baptizo" which means immersing.
Chronologically, one of the first mentions of baptism in the Bible is in Matthew 3:
5 Then Jerusalem, all Judea, and all the region around the Jordan went out to him 6 and were baptized by him in the Jordan, confessing their sins. ...
11 I indeed baptize you with water unto repentance, but He who is coming after me is mightier than I, whose sandals I am not worthy to carry. He will baptize you with the Holy Spirit and fire.
Here John the Baptist is baptizing Jews prior to Jesus beginning His ministry. This baptism was a symbolic washing away of sins committed. It was similar to the washing the high priests did before offering the prayers of the people up to God in the Temple.
Later in that chapter, John the Baptist baptizes Jesus:
13 Then Jesus came from Galilee to John at the Jordan to be baptized by him. 14 And John tried to prevent Him, saying, “I need to be baptized by You, and are You coming to me?”
15 But Jesus answered and said to him, “Permit it to be so now, for thus it is fitting for us to fulfill all righteousness.” Then he allowed Him.
16 When He had been baptized, Jesus came up immediately from the water; and behold, the heavens were opened to Him, and He saw the Spirit of God descending like a dove and alighting upon Him. 17 And suddenly a voice came from heaven, saying, “This is My beloved Son, in whom I am well pleased.”
Jesus did not need to have the cleansing from sins - as He was sinless. Instead, He was baptized as an example for us. (These verses also give us evidence of the Trinity in that all three parts of God are described individually).
Later, at the Great Commission, we are instructed to go and baptize:
19 Go therefore and make disciples of all the nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, 20 teaching them to observe all things that I have commanded you; and lo, I am with you always, even to the end of the age.” Amen.
It is clear that baptism is something the saved should partake in. But, is it required for salvation?
I do not believe it is. First, salvation is by faith and not works. Baptism is a work. Second, one of the thieves at Calvary was saved but not baptized (Luke 23:39-43). There are verses that state baptism and salvation in the same sentence, but if you look at the sentence structure in the Greek, it seems clear that it is not a requirement of salvation (I don't remember the exact verses for this, but if you would like, I can look it up).
Now, for the method of baptism: If you read the first line of the post, baptism should be an immersion, because that is what baptism means. Also, that is how it was done in the New Testament - submersion in the river. So, I do not agree with the sprinkling form of baptism. I also don't agree with infant baptism. I believe baptism is something you do in response to faith. It should be done by the saved as an act of obedience to God. It is a form of public confession of our faith. An infant is not able to make that statement of faith, and therefore I don't believe an infant should be baptized. I know some parents do it as a sign they are going to raise the child in the nurture and ammunition of the Lord, but I think a dedication service is a better fit for this statement.