If you are looking to start mobile development then you will likely be looking at either android or iOS, as these are the most popular mobile operating systems at the moment.
There are only a few languages that are used when developing mobile apps and which one you choose will depend on which operating system you are developing for. If you are developing for android you will want to learn Java, but for iOS you will want to learn objective c or Swift.
While you often do find apps that run on both Android and iOS, if you are planning on doing this I would recommend that you start with one, then once you have gained a bit of skill learn the other.
Something that you should note is that you can develop apps for android on any operating system whether it is Windows, Linux or Mac. iOS apps on the other hand can only be developed on Mac. This is something you need to take into account, if you want to create apps for iOS and you don’t have a Mac you will have to get one, or just make apps for android. Some people have been able to work around this by using a virtual machine to run a copy of Mac, a quick google search should explain how to do this.
Resources for learning mobile development
(as well as the resources above the books from the publishers O’Reilly and Addison Wesley are of high quality, although make sure you get a recently published book so it is up to date).