Reiki Classes and Training has been categorized in different levels. For Reiki Healers, these Levels are Levels 1, Level 2 & Level 3. A Reiki Healer who learns all three levels of Reiki is generally referred to as a Reiki Master Healer.
Since Level 1 is the first level of Reiki, the new healers must be attuned by a Reiki Master before they can practise Reiki. Now a Days there are several books out there who claim that they can give you enough information to become a Reiki Healer yourself, however, that is not true. You need a Reiki Master, or a Reiki Grand Master to attune you to Reiki.
Along with the attunement, you are also taught the Reiki Symbols. You are shown the Reiki Symbol 1 and its use. The Reiki Masters will discuss various cases of healing, show you everything you need to know to get started.
Once you do Level 1, it is recommended that you wait for a few days or may be a week or a month before you do Reiki Level 2. However, as per the trend, people usually do both Reiki Level 1 and Level 2 on either the same day or the same weekend.
If you have any questions regarding this, feel free to ask it here.
Raakesh