If you're hardcore go read the official oracle's documentation for java 8 xd
Just go on youtube and search for java tutorials, you'll probably see one of those "full 383958329309232 hour java course for free
" videos, a good way of learning it is to go trough each stage/part of it, copy the code, understand how it works and then try to build a simple application using that, keep in mind that this is not like a weekend thing and getting on the level where you could actually develop a client with ease is going to take months of improving yourself. After finishing that glorious video you found try to build simple applications and look onto other java apps and see how they are made, or set up mcp using some other tutorials and try to make a client with the knowledge you gained at that point. If you are going to make a client though, don't skid, you won't learn anything and the best outcome of it is you end up like fucking lca or someone where you get 15 a month from a client purchase, everyone is laughing at you and you get doxed.