I have an empty 55 gallon tank and want to change it into an aqua terrarium but I have no idea how to do it. Any help would be appreciated
THANKS

What exactly do you want to keep in it?
What you want to keep will determine how much water and "dry land" you will need, how much and what type of filtration you need as well as which plants you can keep in it.

I want to put a green snake in it and possibly some kind of geckos or lizzards

I had a terrarium years back that was a 55. I used silicone to hold glass partitions in place starting at about 8 inches for the first one and moving up to 10 inches on the second one, leaving a 1 inch gap under each for water flow. I filled it up with about 100 pounds of medium gravel and added water up to the lip of the first partition.
I had a couple water plants in the water and a bunch of random plants in the gravel.
I ran a small pump that ran water up to the highest ground level to feed my rain gutter river bed. The rain gutter was coated in silicone and peppered with rocks.

I successfully kept 3 fire bellied toads, 1 green tree frog, 2 Grey's tree frogs, 1 long tailed grass lizard, 1 fire belied newt, 1 eastern newt, 1 Dojo loach, 4 Cory cats and lastly a fresh water crab.

My land plants were several luck bamboo a hoard of small spider plants and a few swords. My aquatic plants were a hoard of java fern and an amazon sword.

I used a 60 watt heat lamp and a 100watt submersible heater to keep it at around 78 degrees. There were a couple small strip lights with plant happy spectrum tubes.

Given your proposed occupants you would likely want less water.

It would depend what you wanted to keep in the water, mines only a small terrarium (actually a converted BioTube) that houses a trionyx asiaticus soft shelled turtel and 2 columbian running frogs.

As the turtle requires plenty of water and not too deep, theres little water area took up just a hunk of driftwood, the frogs live happilly on the many fake plants stuck to the backof the wall