First, Welcome to RFo. Relax. Have a :beer:. You are in good hands.

Okay, I fought this battle when I first started. There are, in fact, 2 component tools in Revit (as you've found out). The one under the home tab, build panel is for adding components to your model. The one under the annotate tab, detail panel is for adding detail components.

So, yes, you are correct. Detail components add detail to your sections, call outs, and details. But, you have to use the detail components under the annotate tab. The component may or may not be loaded into your project yet. That's why you are getting the wide flange section.

A video would be easier, but I'll try to walk you through adding your top plate in section.
1. Select the component tool under the annotate tab.
2. On the right side of the ribbon, you should see Load Family. Select that.
3. That brings up your library. Go to the Detail Components folder. This is where all of you OOTB detail components are.
4. You will see the different categories. You want Div 06 Wood and Plastics.
5. Then, select 061100 - Wood Framing
6. Then either Nominal or Rough cut, but choose Section.
7. Find your size and load it.

Now it's ready to be placed.

A couple of tips:
You can easily change the size. If you loaded a 2X6, but now need a 2X4, just select it. In the properties palette, you'll see an Edit Type button on the top right. Hit that. Then duplicate. Rename it to 2X6, change the dimensions in the properties. You've just created another detail component with a different size. No need to reload it every time, sometimes it's faster to create your own in the project.

Also, say you loaded everything and you try to place it. What if the wide flange shows up again? Well, in the properties box (again) click on the wide flange at the top. A list will come down showing you all of your loaded components. Just pick the one you need.

...also, when you load your 2x4 from project browser, you need to drag it to your model.

Or right click on it and choose Create Instance.

The 2x category will pop up a box with common sizes - select the sizes you want and it will add them all at once. You can add 2x4, 2x6, 2x8 etc in one go and then just select them as needed. If you need a weird size you can always do what dan mentioned and edit type. If you don't have standard components check out Autodesk Seek. They have all the standard components online as well as quite a bit of manufacturer content, although the mfr content varies quite a bit in quality.

Autodesk Seek

Or use the Detail Component tool and select the detail you want to place from the type selection box...? why make it difficult?

That's how I told him to do it in the first reply. I was just commenting on PijPiwo's post, basically showing the OP that there's several ways to get a DC into the model.

<<<BTW, you like my avatar, Carl? :thumbsup:

...and I was just answering OP question #2 :hide:
Sure Detail Comp tool works the best :beer:

okay, here's where things fall apart - i don't actually have a "detail components" folder. not sure why! maybe it has to do with me having a student version?

it's funny - when i clicked 'load family', i double clicked the 'mass' folder, just to try and open up anything, and when i opened up a simple box i got an error message saying "some or all of the family files you selected cannot be loaded. you must choose family files of the category Detail Items. do you want to retry loading a new set of files?"

so clearly i should have this folder, despite it being a student version!

i'll try to figure this out.

thanks again for your help, i think once i do get things working it'll help a ton.

That's because a mass isn't a detail component.

Make a detail component folder and grab what you need from Seek. I don't know why it didn't install or you can't find it though - that's a bit wacky.

When you get the open dialog, do you have options on the left? One of those *should* be the detail folder if you aren't finding it outright.