underground conduit to power up a garden shed
Where are you located?
Use PVC conduit, the kind you glue at each connection. It is gray, not white. In most areas. if the circuit is not GFCI protected, you must bury 18". If it is GFCI, you can go 12". Those measurements are to the top of the conduit. Local codes could change those depth requirements.
Can you provide some more details about what you're doing? Just running a single circuit? Subpanel? Breaker & wire size?