my first portofolio of game design
The simplest answer? Sit down and start just making things. I managed to make a half hour game in a day using just RTP. Throw together maybe one or two dungeons, a town or two, maybe one more map of exploration (such as a world map or outside map connecting places). I find that just forcing yourself to
do helps a lot with your first.
If you want any sort of plan, I like the 10x challenge I saw awhile back that takes advantage of the trial edition of RPG makers; max 10 maps, 10 events, 10 variables, 10 enemies, etc. It can help keep the game small and restrictions breed creativity.
I know it might sound silly, but don't do anything complicated

The default formula for damage is already quite simple, so just focusing on small, fun ideas and forcing yourself to shelve any complex ideas or anything that takes a lot of time can help a lot.
As for a plot, try to think of a single episode of a larger story. The above note of Dragon Quest 3 is great and that section of the game is probably about that long for a new player. Final Fantasy Mystic Quest also has a fairly simplistic beginning; Ignoring the first cutscene, you are helping a girl try to uncurse a forest, which leads to looking for medicine for the girl, which involves helping a ninja through a desert-themed cave. That's it, easy to swallow and fun story.