Uncal probably means that you have adjusted settings outside of the internal defaults using one of the knobs. Kind of like how being off of "unity" or having changed the default values on a proc amp. Oscilloscopes do this with a "var" knob that allows you do make fine adjustments and when that is used, they'll often display "Uncal" somewhere on the screen to let you know that you aren't using the default settings.
My guess is if you slowly move the variable gain knob, the uncal light will turn green at a specific spot (to indicate it is set to the default spot), or you might just need to press the button right below it to disable the variable gain maybe.
Either way, first step I think would be to attach it to color bars to see if it is already accurate. However you do calibrate it, you'll need one of those. Popular ones would be any of the tektronix pattern generators that usually start with "TSG" in their part numbers. Since this scope is composite only, you might be Able to get away with this inexpensive TSG200:
https://www.ebay.com/itm/365419156344
