Summarising data into a bar chart showing means and standard error

I am very new to R and unfortunately have a beginner-level problem that will hopefully be easy to solve.
I am trying to create a bar chart from a data-frame that will show the mean of my four categories (one bar for each), with the standard error shown as error lines at the top of each bar.
The data is composed of four measurements each for twenty trees, with the twenty trees in the table having it's own row and four columns for the four measurements for each tree (hope this is clear).
I currently have the data in R, and can see the means for my four categories with the summary command, but do not where to go on from here.
Apologies if this is very basic, any help is really appreciated!

You can follow the example at the bottom of this link:

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