For general electronics soldering, get 60/40 wire solder with rosin core. 800 µm is a good general purpose diameter, although in some cases you may want smaller for fine work.
Note that in some parts of the world solder containing lead is not allowed. None of the alternatives are as nice a lead/tin solder. As a hobbyist, use lead/tin solder if you can. If you live in Europe or California or anyplace else this is banned, don't try to sell equipment built with lead solder. For personal use, don't worry about it if you can get it in the first place.