HI @Julia.m1234,
Welcome to the RStudio Community Forum.
To check that your Rmarkdown setup is working correctly I suggest you open a new Rmarkdown example file from the "New File" menu icon (leave the HTML botton clicked on). Don't make any edits, just save this file to a suitable filename e.g. testing_markdown.Rmd (note the file extension must be .Rmd). Now click the "knit" button. The markdown HTML output should appear in a Window (including a simple graph). If you don't see this you will need to post the errors that you see in the "R markdown" tabbed window in RStudio.
HTH