You should read the paper Acme: A User Interface for Programmers, which is in your file system as /sys/doc/acme/acme.ps if you are using Plan 9.
You can also find more info in the unoficial Acme website.
When you start Acme, you see several windows layered into two columns. Above each window, you can see a "tag line" (in blue). The first thing to notice is that all the text you see is just that: text. You can edit anything at will.
To take an example, Acme usually displays the contents of the directory where it was started. Create a new window and start Acme on it. Now, assuming you started Acme while your current directory was /usr/glenda, you can see a window with contents
bin/ lib/ tmp/
The "contents" is called "body" in Acme. If you look at the window's tag line, you will see that it contains
/usr/glenda/ Del Snarf Get | Look
That is just text.
Each mouse button (1, 2, 3, from left to right) does a different thing in Acme:
You should know that you can use double-click with button 1 to select words (a double click near the end/beginning of a line would select the whole line). Besides, once you have text selected, you can simply click on it with button 2 to execute the selected text. A single click of button 2 would execute the word clicked as a command.
Now let's pay attention to the tag line once more. As you can see, the left part has a path. That is the name for the window and shows also the directory for the thing shown (file/directory/program output). When you execute something using button 2, the current directory for the command is the directory shown in the left part of the tag (if the thing shown is a file, its directory is used).
As you saw before in the example, there are windows labeled "/dir/+Errors", that is where Acme shows the output of a command executed in "/dir".
Another thing you can see is that tag lines contain words like "New", "Del", "Snarf", etc. Those are commands understood (implemented) by Acme. When you request execution of one of them, Acme does the job. For example, click with button 2 on "Del" in the "/usr/glenda/+Errors" window: it's gone.
The commands shown by Acme are just text and by no means special. Try to type "Del" within the body of the window "/usr/glenda", and then click (button-2) on it.
These are some commands understood by Acme:
Acme likes to place new windows itself. If you prefer to change the layout of a window, you only need to drag the layout box at the left of the tag line and drop it somewhere else. The point where you drop it selects the column where the window is to be placed now, as well as the line where the window should start. You can also click the layout box to enlarge its window a small amount (button 1), as much as possible without obscuring other tag lines in the column (button 2), and to fill the whole column (button 3). You can get your other windows back by button-1- or button-2-clicking the layout box.
Acme allows you send output and take input from other programs. One of the most common applications of this is using commands like sed(1) to edit your text. If you wish to change "foo" to "bar" in a section of text in acme, you add the following pipe command to the tag bar of the acme window wou wish to modify.
| sed 's/foo/bar/g'
Note the | operator. Now you can use mouse button 1 to select a section of text by highlighting it. Now use button 1 again to highlight the entire command shown above (including the pipe operator). This highlight appears in blue. Now button 2 click on the blue highlighted command, and it will be applied to the selected text in the window below. Some common utilities to pipe to/from are sed(1), grep(1), tr(1), and sort(1).
This is mostly what you need to get started with Acme. You are missing a very useful feature: using combinations (chords) of mouse buttons to do things. You can cut, paste, snarf, and pass arguments to programs using these mouse chords. You can read this in the acme(1) manual page, but it's actually extremely simple: Select a region with button 1 but don't release the button. Now clicking button 2 deletes the selected text (putting it into the snarf buffer); clicking button 3 replaces the selected text with the snarf buffer. That's it!