Believer's Baptism is for anyone who requests it, after they have come to the realisation that they have become a Christian. Not all Christian churches will follow this procedure, and the new believer may have to search around a bit before he finds one that will oblige1. It is not every believer who will see any need to be baptised and the 'correct' way to approach baptism is just one more thing (among many) upon which Christians are not in complete agreement.
What Happens - and Do We All Get Wet?
Any body of water more than a couple of feet deep will do, although most churches who practise this form of baptism will possess their own specially-constructed baptistry (think of a very small swimming pool without the diving board and with no lifeguards, and you just about have it). Outdoor baptisms in rivers, lakes or even the sea are sometimes performed for the real bravehearts.
The 'candidate' (as he or she is often called) usually has to answer a few questions, to publicly confirm that they understand pretty well what they are doing, and then they are momentarily plunged completely underwater2 in 'the name of the Father, the Son and of the Holy Ghost' by a member of the church (most often the Minister, if there is one). The questions will ascertain whether the candidate is willing to try to renounce sin and if they have accepted Jesus as their Lord and saviour.
Very often at a baptism the candidate is encouraged to speak about their own Christian experience so far, which has led them to 'take the plunge' in this way.
Towels are generally provided.
Just Why Do They Do It?
The main reason is really to show solidarity with Christ. Symbolising the spiritual rebirth or awakening which has taken place in their lives, this event is a fairly dramatic but convenient and reasonably gentle way for believers to announce in public their new found faith in Christ. It is also a way of demonstrating to others their growing determination to follow his leading and example.
What Other Symbolism May be Involved?
The use of water in this way is said to symbolise the blood of Christ being used to wash the believer clean (up until their acceptance of Christ, their life had been stained by being marred by sin).
Another idea which is often worked into explanations of baptism is that of spiritual rebirth. The desires for the former largely selfish human nature to go down to death in the water/blood and for a newly made being (who hopes to live only for God's glory) to rise up are being acted out. Of course, everyone knows that it is really the same person who re-emerges (only wetter) but it is hard to think of a better practical way to simultaneously represent the ideas firstly of being washed, and secondly of becoming a new creature with an entirely new outlook on life, the universe and everything.
The fact that Jesus himself rose up after dying and going down to the grave is echoed here, too.
Why Else Do They Do It?
Baptists will also tell you that Jesus has commanded them (via the Bible - see Matthew 28: 18-20) to get themselves baptised.
Many people will use their own baptismal ceremony as something to invite friends and relatives along to in the hope that they too might develop an interest in Christianity. Stranger things have happened but probably not very often. Many churches combine the baptismal service of a believer with his acceptance into full membership of the congregation.