I have been developing a few web apps and one Slack app for Eve.
If you want to develop web apps you want to go PHP, node.js or Python. Since you already know Java, I would recommend PHP, since the syntax is similar and its supported by almost any web server. (also while learning the language stackoverflow is your best friend 
)
Standard stack would by xAMP, so Apache, MySQL and PHP. That works great both on your dev machine (e.g. WAMP for Windows) and is pretty much standard for any webserver (e.g. LAMP with LINUX).
To make your web app user friendly you want to add some JavaScript, usually in the form of proven GUI library. I can personally recommend jQuery and Bootstrap. Or you can go further and look at Laravel.
For testing you web apps on your local dev machine with ESI authentication (e.g. SSO login) I recommend using ngrok, which allows you to expose your local webserver to the Internet in a secure fashion.
Last but not least I can recommend joining the tweetfleet Slack for help with any Eve API and web apps related topics. There you can directly talk to the CCP devs for ESI and also find many 3rd party devs.
Here is an example for one of my web apps: Thera Connect. It finds the best routes between two solar systems in Eve using Thera as hub.