I'd like to explore some things regarding web CMS's in java and I would like to make an own one (very light), but I pretty much have no clue in how to begin. I'm used to java as a language and I've made some similar things in PHP, but I've never used servlets or java techniques in this matter.
Is there any "tutorial" or similar in this subject that I could have as a reference? Or any open source project that is very light with low dependencies?
Here is a list of (mostly) Java CMS tools that you could investigate. I recently looked for some CMS tools myself and found this page useful.
Note that CMS tools are built on top of other frameworks so you might have to investigate a bunch of other technologies, not just servlets. There were also some similar questions posted on SO by people trying to get advice on choosing a CMS. You should check them out.
About tutorials, I can't really say. I was looking for an already built solution and extend on that, but if you want to "disect" some of the available open source CMSs to see how they work, first check the amount of documentation available and the community (not all of them are fully documented).