Vegetation index using the Raster Calculator

Click the Open Data Source Manager and then Add raster layer and select the file /geodata/lucc/s2/Subset_S2A_MSIL2A_20170619T_MUL.tif.
Check the rank and spectral range of bands in the multiband file (e.g. see table Changing Raster Layer Style).
After clicking Open, the raster layer appears in the QGIS canvas.
Click Raster --> Raster calculator.
A powerful normalization is realized by the normalized difference vegetation index that forms the basis of most vegetation indices $NDVI = (Nir - Red) / (Nir + Red)$. It can be implemented using Raster Calculator as seen in the following screenshot:

Define path and file name of the output layer. OK.
Map visualization: Open the raster layer properties by right-clicking the ndvi raster in the TOC selecting Properties, or by simply double clicking.
Select the Style tab. Under Render type select Singleband pseudocolor.
- Open the In the pulldown menu of Color ramp by clicking on Select All color ramps --> RdYlGn. In the Mode select Equal interval. Set the number of Classes to 10 and click Classify.
- To finish, click Apply (see Fig. A).

Spectral indices using Processing Toolbox

A convenient module for calculating one or several spectral indices at the same time can be found in the QGIS processing toolbox.

Type Radiometric into the search field and find the OTB --> Feature Extraction --> Radiometric Indices) tool.
Open it with a double click.
Assign the bands in the multiband file to the correct spectral sensitivity.
If you want to calculate a single spectral index type in the text field Available Radiometric Indices: "Vegetation:NDVI"
If you want to calculate several spectral indices at one time and store them in a multiband file click Help to find definitions of available spectral indices in three categories (vegetation, water, soil) and relevant channels of the input multiband image.
Click Run.
In the text field Available Radiometric Indices type with apostrophs:

"Vegetation:NDVI Water:NDWI2 Soil:BI2"

Figure A: False color composite RGB = 4,3,2	Figure B: Single band NDVI