Hi, Henry. This is a very broad question. The answer can be summarized as "Pentaho is a great BI suite". You have several options to start with Pentaho, it all depends on your interests.
If you are interested in BI in general, you could start by reading Pentaho Solutions, one of the Pentaho books available.
If you are interested in learning an excellent tool for manipulating data, then Kettle / PDI is what you should learn. PDI is the Pentaho tool used for data manipulation, and is the main subject of my book,