If you are new to the field, take your time to study and compare some opinions and approaches. There is no standard way of fusion, so it also depends on what you want to do with the data after the fusion. I listed some references to fusion approaches here: Fusion of S-1A and S-2A data
It might be suitable to convert the SAR data to db (also right-click on the SAR bands for this) because this creates a more suitable distribution of backscatter values. Explanations are given here: dB or DN for image processing? and here Classification Sentinel-1 problems with MaxVer
Once they are in your stack you can create an RGB image by right-clicking on the product and selct “Open RGB image window”. This lets you allow to place colors on different bands and shows you their different information content.
Some fusion methods also can be done in the band maths tool (right click > band maths)
PCA is available under Raster > Image Analysis > Principal Component Analysis.
Maybe also an unsupervised clustering is an option to you (Raster > Classification > Unsupervised Classification)