If you buy a new larger filter, then take all the media (that's all the stuff inside, usually sponges, but sometimes ceramic rings or 'bioballs' made from plastic) out of it, put the media from your current filter in there (chop it up if needs be to make it fit) then fill up the space with the new media (again, cut down if needs be). Tat's better fro transferring any bacteria over than running the two filters together.
The fish are reluctant to come out because they don't feel safe. Oddly enough, the more full of plants and caves and hiding places your tank is, the more you'll see your fish, as they'll know they have hiding places nearby they can swim to in a hurry. You and I know there are no predators in our tanks but the fish don't!
Unless you can go to your LFS for water testing every day, you'll need to do daily water changes of at least 50% to keep the ammonia and nitrite down. When you can get kits, for those two at least (you needn't worry about pH, hardness or nitrate at the moment) you might be able to do less water changes, but there's no way of telling without testing (you can't see or smell ammonia or nitrite, until they get to very high levels).