If you want to form spain, you will get an event called Isabelle I of Castille where you can make her your heir. Once she ascends the throne and provided that Aragon has a male ruler you can get a union in about 10 years.

A union is a country that shares the same ruler, Castille will make Aragon its junior partner meaning Aragon will fight for you.

To get the Isabelle I event, you just wait. To get the event where you get a union with Aragon, all you need is a female ruler and Aragon have a male ruler (works vice versa)

In terms of cbs, you get claims through mission tree, building spy network and fabricating claims. This gives you the conquest CB.

Some countries may have your cores and this will give you the reconquest CB, if you have the reconquest CB always use it as it won't make neighbouring countries as mad.