Lesson 1: How to create an effective Beginners Deck

Step #1: Pick A Theme

To have a good theme means to make sure the cards in your deck can support one another to make a combo. You don't want a random mishmash of cards that may look cool but in reality don't do much together. An easy way to think of a theme is to select a Yu-Gi-Oh! archetype. Check this link out -->  Yu-gi-oh archtypes
It is an entire list of all the archetypes out there! For example, an "elemental hero" deck is an archetype.

Step #2: What is your goal?

All decks in a goal to win. Whether you are trying to reduce your opponent's lifepoints to 0 or deck them out, you need to have a goal in mind. What do you want your deck to accomplish?

For example an elemental hero deck wants to accomplish fusion summoning to bring huge fusion monsters on the field to overpower your opponent's monsters, thus dealing damage.

At the same time, a good elemental hero deck would want to protect his fusion monsters. Another element to think about is having the right resources to do fusion summoning. In order to fusion summon, one must have the spell card "Polymerization". There are of course other cards that successfully fusion summon, such as "Future Fusion" and "Fusion Gate"
.
Step #3: Main Deck Part 1 - Choose the monsters you want to command!

What monsters are you going to bring out to the playing field? What do you want them to accomplish? This is what you have to question as of now.

Take advantage of effect monsters and their epic pwnage abilities. Back in the day, normal monsters were fine to use but in today's Yu-Gi-Oh! format normal monsters are a setback. You need to use effect monsters to your advantage. But of course, if normal monsters are a part of your theme, use them! Certain variants of "elemental hero" decks have normal monsters because they are fusion material monsters. (e.g. "Elemental Hero Flame Wingman" has 2 normal type fusion materials, "Elemental Hero Avian" and "Elemental Hero Burstinatrix")

As appealing as high level monsters are with their sky-high attack and defense stats and supah powah abilities, you may want to put a limit on how many of them you put in your deck. Your average build will have 2 or 3 high level monsters in the main deck. Any more, and you will risk having a dead draw! Also, make sure the high level monsters you have are easy to special summon, if possible. Having to do 2 or 3 tributes is time-consuming and not efficient. Of course you can do it if you wanna, but special summoning the high level monsters is something you should give some thought into. For example if you play a deck with lots of light and dark attribute monsters, you may want to include "Black Luster Soldier - Envoy of the Beginning". It requires absolutely NO tributes; just you gotta banish 2 monsters from your grave. That is an example of a high level monster that can be special summoned.

I should mention that most good Main decks have 40-44 cards in them. Out of that, 18-22 should be the number of monsters you should aim to have in your deck.

You may want to give staple monsters some thought. What are staple monsters, you ask? Why they are cards that are beneficial to almost any deck. Here's a link to check them out. http://yugioh.wikia.com/wiki/Staple

Step #4: Main Deck Part 2 - Choose the spells you wish to cast!

Spells are absolutely neccesary. There are a wide variety of spells to choose, but which ones should you include? First things first, add the staple spells. "Dark Hole", "Monster Reborn", "Heavy Storm" and "Mystical Space Typhoon" are spell cards that 99.99999999% of decks benefit from. Next up, choose spells that support your theme! For the elemental hero theme you want fusion cards, and you also want cards that are resourceful like "Reinforcement of the Army" to get the correct fusion materials that you need for your fusion summoning.

Step #5: Main Deck Part 3 - You activated my trap card!

Most decks use trap cards, but I've seen that some decks that don't use trap cards work perfectly fine too. imo use trap cards. However if you choose NOT to, please insert "Treacherous Trap Hole" into your deck. That card can't hurt any deck that has 0 trap cards. Another idea is to add "Royal Decree". This continuous trap card negates the effects of all trap cards - which is a great way to disable some of your opponent's cards. Again make sure to include the staples, "Mirror Force" is one of them. Look for traps that support your theme. For example "elemental hero" decks can have the trap "Hero Signal". Trap cards in your deck should be 8-10.
Also, look into less used traps, many good cards are just forgotten!

Step#6: Extra Deck - Summon your most powerful monsters!

In the cases with most decks, A POWERFUL EXTRA DECK IS JUST AS IMPORTANT AS A POWERFUL MAIN DECK. This is because while cards in the Main Deck are useless unless they come to the hand, Synchro and Xyz Monsters can be used from multiple situations. For instance, you have to draw Foolish Burial to use it, but Lavalval Chain can activate the same effect as Foolish Burial at any time, as long as you have 2 Level 4 Monsters.

Good duelists run multiple copies of powerful cards in the Main Deck to increase the chances of drawing it. However, how many copies of Stardust Dragon you run obviously does NOT increase the chances of summoning it. It does, however, increase the chances of summoning a second copy from 0% to 100%, and this is why I said “save for several irregulars”. If a deck constantly wants to summon over 2 copies of the same Extra-Decked Monster, you should run multiple copies of it. Otherwise, run 1 each of everything. There are only 15 slots for the Extra Deck, and the more options you have, the better your chances of victory are.

Then, most of you will be thinking: What are the best options?

So, for those of you thinking like that, here’s a list of the several best Generic (Generic = “Not having summoning requirements except Levels”. For instance, Constellar Pleiades and Chaos King Archfiend are not generic, while Maestroke the Symphony Djinn and Black Rose Dragon are) Xyz and Synchro monsters for each Rank/Level. Cards already released in the TCG are bolded. Others are OCG only. The higher the place in the section is, the better it is in most decks (for instance, in Rank 3s requiring 2 materials, Wind-Up Zenmaines > Leviair the Sea Dragon > Number 17: Leviathan Dragon > …etc.) However, this is (i) just my personal opinion, and (ii) does not go for all decks. This is just some advice, and does NOT go for everything. Also, Xyz Monsters requiring over 3 materials are generally looked down upon, but depending on the deck, it could be more powerful than a weak Xyz Monster of the same Rank with 2 materials.

Tips:
Keep your deck around 40 - 45 cards if possible. This will make your deck more efficient and make it easier for you to get the cards you want.
Remember that XYZmonsters have ranks not levels.
You can add cards to your side deck to allow you to swap cards around or add more cards during siding.
Learn your deck and practice with it before you enter a ranked duel.