You have to be really brave to be a goalkeeper. People say it's the worst position but if you ask me it's one of the most fun and best. You have to be able to dive fast and effectively. Height matters, 5'6'' is really short to be a goalkeeper, if you're gonna play a proper match in a little league you guys made up I think it'll be pretty bad for you mainly because your really short (don't take it as criticism). if you really absolutely wanted to do it then go ahead, 6 months is a long time to improve your reflexes. Also remember, being a goalkeeper requires split-second decision making (e.g when to rush it ball out of goal, when to make a tackle when your defenders are beaten). Constant practice will make you better, practice with friends or family 2-4 times a week and you should be fine.

You must learn to come out of the box and make yourself large while making the goal small.

Well height does make quite a large difference but all that means is you have to dive acrobatically (like Casillas). I would start with normal simple dives until my natural instinct of not trying to fall is gone and after that you have to start to dive further and quicker and work on your landing. Hope this helps