Probably you want to use surface reflectances. So you should do an atmospheric correction first or directly use the L2A data from the SciHub. @Abraun suggested this already.
If you want to do it on your own you have several options. You can use Sen2Cor, Sen2Three, or iCOR.
See the Third Party Plugins on the STEP web page.
If you have this data you need to resample the data to the resolution you would like to use.
You can take the standard resampling operation (Raster / Geometric Operations / Resampling). It is faster but does not consider the special characteristics of the view angles of S2MSI. This can lead to some stripes at the camera borders. This effect is negligible over land but over water, with a low signal, it might need to be considered. Judge yourself.
You can also use the special S2 Resampling (Optical / Geometric / S2 Resampling Processor). It does the handling of the view angle better. But also not fully correct. This is currently not possible with the current data (An update is planned to fix this). Check also the help page.
