You've opened a pretty significant can of worms. The best answer I'd give here is that you'll get different answers depending on who you ask. Though I might add the following:
The historic understanding of the Christian Church is that a baptism is valid if it is in water, and in the name of the Father, Son, and the Holy Spirit. The most ancient form in which baptism was done is triple immersion; when full immersion was not possible affusion (pouring) has been acceptable; this is recorded exceptionally early in one of the earliest non-Canonical Christian texts, the Didache, written probably sometime between the mid and late 1st century (making it possibly older than many parts of the New Testament). In the West pouring eventually became normative, while in the East immersion (that is, triple immersion) is still how Christians baptize even today. Single immersion is almost unheard of, though it did happen in a few places. The validity of a baptism is not the form--pouring, immersion, etc--but that it is in the name of the Father, Son, and the Holy Spirit, and with the use of water. Thus "Jesus' Name" baptisms are considered invalid and illicit; and in certain cases when a group baptizes in the name of the Father, Son, and Holy Spirit but does so with a rejection of the Trinity it is an illicit baptism.
Otherwise, even an atheist could perform a valid baptism.
As noted, this is a more historic, traditional perspective; and you will get many different answers.
-CryptoLutheran