Different Computer

[GreenT]

The computer that I use to play games, such as Minecraft, is really laggy and is not meant for streaming. When I did try to live stream with it to YouTube, it would switch frames every 10-20 seconds, and it was very poor quality even though I have a 720p screen. I was told it was the graphics card and the CPU, but I am still unsure.

1. What is making it like that during my streams?
2. What is a recommended computer to get, along with a monitor? (720p is fine)

Any help is greatly appreciated!