See learn.sketchup.com for a series of video tutorials. That should get you started. Come back to the forum with questions when you need further help.
Always leave the default tag as Untagged when drawing a new object so your edges and faces are always Untagged. Make it into a component as soon as you finish drawing it - triple click to select all of what you have just drawn then tap the letter g to Make Component. In the dialogue box that then opens, give it a meaningful name. Ensure the box is ticked that says ‘Replace selection with component’ in the Make Component dialogue then click the button to make a component of what you have just drawn.
Components and groups are containers that stop your geometry from sticking together - they stay independent of each other.
If you are making parts for 3D printing, I’d suggest using metres as your units (Window/Model Info - Units) but use them as if they were mm, to avoid problems with small edges. Then scale down by 1000x for printing.