when i go onto a world it has a glitch attack

about a month ago i got my computer back from being fixed and installed minecraft.

what i expected to happen was a normal minecraft world to build and have fun on.

*What actually happened was chunks would appear above and below me while the original world would be invisible. there would be trees beneath me.

1. i closed out of a normal world then opened another
2. ...then every step i took it would go crazy