How deep to I have to go, do I need conduit, what gauge do I use for a 50A service line to tha garage

For residential, you can use either:

#6 - 3 wire plus ground UF cable buried 18 inch deep,

Or,

1 inch conduit buried 18 in deep with 3 -# 8 with THWN or XHHW insulation and 1 #10 ground.

If the soil conditions are rocky, I suggest using PVC conduit.

Check with local codes for any preferences, and if a warning tape buried below grade and above the raceway is required.