Should I put the table/figure/diagram first or the explanations of them first?

Thanks : )

I would usually present the table or figure before I describe it. This is just a suggestion of course and is my personal choice rather than a particular rule or convention - it really is down to your own editorial judgement and what is typical for publications in your discipline.

You can check the "ACM Word Template for SIG Site"...

In Page 2 column 1, you can find this section...
4. FIGURES/CAPTIONS
Place Tables/Figures/Images in text as close to the reference as possible (see Figure 1). It may extend across both columns to a maximum width of 17.78 cm (7”).

In Page 2 column 2, you can find figure 1.

That is, the figure or table can come later...
Anyway, this is my interpretation of ACM guideline.
If figure 1 appears first, the reader may not be clear of its purpose.
But psychresearcher's suggestion is also possible for other writings...

To clarify, I would always make reference to the figure/table etc. first in the text. e.g. Figure 1 displays data from Experient 1. Then present the figure, and then give a brief description of what the Figure shows - but as I mentioned earlier this is just my preferred method and may not be appropriate or indeed what is suggested for this particular conference submission. I would just suggest you adhere to all the guidance on the conference submission guideline.