A good starting place to begin might be to look at the Sentinel Missions with Sentinel 1, 2 and 3. You can see details here.
This data is freely available for download from a few platforms but I would recommend the Rocket Project from Mashup for any new people to remote sensing, which can be found here. An important thing to consider is the temporal resolution of the data because this may cause unwanted and unexpected results after fusion implementation.
Sentinel 1 uses radar, Sentinel 2 is multi spectral and Sentinel 3 has a wide range of instruments but the one you may be interested in is for land surface temperature.
You can then use the Sentinel Application Platform (SNAP) which is free to download to help you pre-process them and can be found here..
Other than that it is just a case of choosing any other software you might want to use and having a look online and in books for some detailed workflows.