Hi Lorena. The easy answer is that, if in doubt, use "though".
Whenever you need to use "although", you can also use "though" - yes, it can sound less formal sometimes, but this is not a big problem.
Do you want a longer answer? I could give you one, but I think you can also find them on the internet: http://forum.wordreference.com/showthread.php?t=30981