That's a nice solid looking Mixte framed bike you have there, and the fact it has Reynolds 531 tubing makes it really worth it to fix up. It looks realtively clean but it also looks like it can use a general overhaul to get it to good riding condition.
First, buy a good set of tools (make sure you get one that includes allen head wrenches) and make sure that you have a good range of metric sizes. Start wrenching away, but make sure you keep parts together and in order. Little andwich baggies would be useful for storing and keeping parts together. buy a big can of WD40 and a bottle/tube of bicycle grease and lubricant.
Best way to learn about your bike and how to fix/restore bikes in general is to just get into it carefully and learn as you go. Please be aware though, that there are a few parts that will require special tools and methods to disassemble such as the bottom bracket, crankset and headset, but you can get the proper tools for these if you would want to deal with them from a good bike shop. The bikeshop can also be your source of help as last resort if you cannot figure out how to handle certain parts of your bike.
lstly, get yourself a good book on bicycle riding and maintenance there are many good ones out there that have been written through the years, but make sure you get one that covers older/classic bicycles because bicyclces and their parts have evolved tremendously in the last two decades.
Have fun with the restoration project on that Mixte!

Chombi
84 Peugeot PSV