Help me beat an 11 year old!

Well, honestly there isn't really much that I can do to help you other than tell you that you are good, just pull something together, and remember to make it as consistent as you can. I personally like blue cards for their utility purposes, green for their power and red for the spell damage.

start by deciding what you would like to have your deck do or just have a general theme. Limit yourself to a one or two color deck for consistency. I find blue to have the best all around utility spells. So if you decide to make a bi-color deck consider blue as one of the colors. It should work well with any other color you put it with. Only approach a tri-color deck if you have cards that compliment each other.

Keep the mana cost down. I usually end up with an average converted mana cost of 2.3 -2.75. I personally prefer to keep my card count down around 60. Very rarely do I go over that cap. My draw consistency is diminished for each card over 60 I have in a deck and adding colors to decks reduces the consistency as well but it is manageable.

Most players will tell you to run X mana in a deck or what have you; I have found some success with a 2:1 ratio to start. That is 2 playable : 1 mana. This works fairly well in some cases as you tend to have a 1 in 3 chance to draw a mana. I have made some ok decks simply dividing the composition into thirds. (20 land 20 creature 20 spells)

A common rule of thumb is to rock out with the power nine. (9 play sets, the rest mana) this will give you 36 spells and 24 mana.  This is a consistancy guideline more than anything else.  I start decks like this and end up changing the deck around as i get a feel for how it is working.  I made a Izzet (red/blue) deck with 12 creatures, 18 mana and 30 instant or sorcery spells. I scrapped that idea after a few months not because it didn't work but because I wanted to use the cards in other decks.

When making a deck (since I play kitchen table magic) I do not need to think about what format my deck is but that may be a factor for you, and will limit your choices so read up on the formats and see whats what.

I would like to touch upon the mana balance in your deck. If you have a one color deck then there is no real issue other than how many lands you wish to include. Too much mana and you will become flooded and too little you will have a drought. In a two color deck I will start with an even amount of land for each color. (example: 12 forest 12 swamp)  but really this should be decided by your devotion to a color. Double and triple color mana cost sometimes gets a bit tough with multicolor decks and as such should be a factor in creating your deck. (example: if i have more green mana symbols than black I would drop a few swamps and add in a couple green. Simply adjust your mana base to suit your decks needs. For me this has always been a trial and error thing.

Lastly you need to be up on all the new rules. Wizards of the Coast has changed up some key play mechanics in the last 15 years, I have only been playing for 2-3 years but in that time I have learned mana burn no longer exists. The "legendary rule" has changed from allowing only one legendary with the same name to be in play at a time; to each player can only have one legendary that shares a name. Every other rule I have either looked up when I need a clarification or in my free time I read through some of the rules, there are a bunch.

Hope this helped a little Good luck and Happy travels Plainswalker.

Looking briefly at your inventory I would say to build a Blue Black shadow deck. Include Mind Twist because it's awesome (at one point it was even vintage aka type 1 restricted), You should play that portal Command of Unsummoning just to mess with him (instant speed Sorcery), Dauthi Horror is still good, Diabolic Edict is still good, Dauthi Marauder is ok, Dauthi Ghoul seems alright, of course counterspell is good and Capsize with buyback can really be obnoxious.  Dismiss seems playable also.  Alternatively if you want something consistent I do see a playset of Mogg Fanatic, I bet you could build a sweet red deck.

https://deckbox.org/sets/779642

The idea was to put out lots of low cost goblins as fodder,  and then blast the heck out of him with instants combined with furnace of wrath.

The deck is a little big -- what would I take out. Is this strategy even feasible?