First, you will want to 'Select Features by Area', you can select whichever features you would like to buffer using this which is located on your toolbar. This is good if you are just selecting a few features to buffer, but if you are dealing with a large dataset and you are trying to select hundreds of separate features to buffer then open up the attribute table and 'Select Features Using an Expression'. With this you will want to set up a query or expression to select features by.
Lastly, use the 'Create Buffers' tool. This tool gives you the ability to buffer by whatever distance you would like. The 'Create Buffers' tool is very useful ain QGIS, and being able to use this tool properly will help you out in your spatial analysis. I have used other buffering tools in QGIS, but 'Create Buffers' located under MMQGIS works the best.
The first photo is a screenshot of the settings you can adjust for this buffer tool. This is where you will want to specify 'Selected Features Only'. Sorry, this screenshot I took wasn't the settings that I used for the screenshot of the buffers. I made sure to check the 'Selected Features Only' box.
The second photo is a screenshot of a quick buffer that I did to show you the two points that were selected to buffer.