Hey,
Partly depends on budget, but in general:
1) SSD (Windows, apps)
2) HDD (Gaming, Windows backup, videos etc)
Games will load faster on the SSD (approx 1.5X to 3X) but for most games that's not a huge deal. Maybe 10 to 30 seconds savings at beginning of game and savings later with level loads.
It does matter for games like SKYRIM that do a lot of loading (buildings, jumping around map etc).
a) Install Steam (like normal to C-drive)
b) create a folder on HDD from Steam settings-> Library.. (i.e. "E:\Steam")
c) when installing a game choose the E-drive folder (HDD) usually.
Enough space for several games in addition to the OS etc. At least 100GB likely for games on a 250GB SSD. Can also add another SSD later if needed and put another Steam folder there (can have on per drive). Moving games between folders is simple: can create backup of game, delete game, restore then choose different game folder.
Examples:
http://pcpartpicker.com/part/samsung-internal-hard-drive-mz75e250bam (install Samsung Magician as well)
and
http://pcpartpicker.com/part/seagate-internal-hard-drive-st2000dm001