BudgetAudiophile

I'm a complete newbie in regards to Audio Hardware, so I would like a guidance with the setup that I want to build. I'm planning on having an Audio Interface plugged into my computer where I can connect some headphones and a mic into it. I want the ability to use my headphones as a monitor to the mic connected to the audio interface at the same time that I can hear my computer audio. In the future I'm also planning on buying an electric guitar and connect it to this Audio Interface and use the Headphones as a monitor for the guitar as well. So, I would have the headphones playing audio from multiple sources: the computer, the mic and the guitar all at the same time. What do I need to buy to achieve this? Some recommendations to a good budget Audio Interface, Mic and Headphone would be greatly appreciated. The only thing I currently have is a cheap JBL headphone with a green/pink connector that I'm not sure would fit this new setup that I'm planning to do, so I'm willing to ditch it if necessary.