worktop is quite heavy.
You can support it wherever it touches the wall (back and ends) by fixing a wooden batten to the wall. 50mm square would be fine. Ends that are not against the wall, and long runs of a metre or more, need to be supported with either a leg or a panel. You can get chrome legs with adjustable feet. Panels are usual between the spaces where appliances will go. Provide your switches and outlets for appliances before fitting the worktop.
Use a long spirit level several times to verify that the supports are at the same height. Don't measure from the floor as it might not be flat.
It is an advantage to have a very slight slope away from the wall so that spills and drips will not run into the gap where they are much more difficult to clean up.
The legs or panels must be fixed to the floor so they don't topple when somebody kicks them. Use feet on the panels. Chipboard or MDF will fall apart if it is resting on the floor and it gets wet from spillage or mopping.
The battens, panels and legs must be screwed to the worktop from underneath so it doesn't fall or slip, especially when you climb on top of it to change a lightbulb.
The walls and corners are unlikely to be straight or square. You can hide small gaps with silicone sealant colourmatched to wall or worktop, or larger ones with upstand that can match the worktop. Chipboard or MDF upstand will fall apart if it gets wet from spills or leaks. Tiles will crack away from the worktop if you use grout, silicone will last longer.
You can get legs and KD fixing blocks from places like IronmongeryDirect. If you use modesty blocks they will not save time during build, but you will bless them if you ever need to move or alter or replace the worktop.