I would like to install a 30 amp subpanel in my detached garage. I plan on having a 30 amp breaker in my main panel fed to a main lug subpanel with one 2-15 amp single pole tandem breaker.

Questions

Can I use 10/2 uf-b cable?
Do I have to have a grounding pole in the ground?

You would need 10/3UF.
Yes, you need a grounding electrode of some sort.

Comments/questions:
- WHY only 30A? Running a slightly larger wire now will only benefit you in the long run.
- WHY a SP tandem breaker? Why not just use standard SP breakers?
- You need a form of disconnect at a detached structure. A main breaker panel would be easiest, or you can use a back-feed breaker with a retention device.
You can get away with having six breakers or less, but if the panel can hold more than six breakers, or will accept twins, I would install some form of disconnect.

Why would I need 10/3? On the load center, a Square D QO2L30S, 30 amps max, it says on the wiring diagram if only using one breaker, 120Vac/ 2 wire, a hot , a neutral, and a ground to a ground bar that I had to purchase separately? Is this diagram wrong?

- I have no need for anything more, just lights and receptacles. All my other tools are gas driven(welding machine, air compressor, etc.)

- I was going to use the tandem to get away with having to use 10/3 for two breakers(unless the diagram is wrong). The tandem was going to be 2-15 amps, one for the lighting and one for receptacles.

I want to make sure that the wiring diagram is right before I continue.

For the life of me I can't understand why someone would run a 120v feeder to a detached structure for the sake of saving one conductor, but OK. You can do it that way if you like.

Since you are running a feeder to a panel in a detached structure you still need a grounding electrode though.

Tyvm stanfortyman!