Looking to create a Working Gui for a species mod and would like some help.

So i am rather new to this and i downloaded MCCREATOR with an idea and no actual idea how to code and all that technical stuff.

the basic idea is there is a start up menu on the screen when you open up a new world, then you can choose a list of species. for now all i would like to do is focus on what i think would be the easiest species. Mushroom Men!

all i need settled is how to get the select abilities they have working. The first ability. the ability to go into the ground and disguise as a mushroom. of course what i mean by that is that the players body is forced into dirt/wood/stone/sand and the head is fully kept above those blocks. while disguised they can release poison fumes damaging mobs, and turning villagers into mushroom men NPC's. when an entity is on the disguised Mushroom man, they can leap out and cause a 10 block radius of damage to nearby entities like a harming potion.

mobs don't naturally go towards the player when disguised, but if they get within 2 blocks of the disguised they will attack. while disguised you gain hunger and health rapidly. by killing the mushroom men mob, you can get a special mushroom which i'm calling the "Mistreated Morsel". you can use this in a potion that gives non mushroom men players Morsel Powers!

all i need help with is making that new world gui and the Morsel people abilities setup for players

this does seem really complicated reading it out so i don't expect much help lmao.