(I’ll be using the commander format as an example for explanation)(Skip to 2nd paragraph for 60 card formats like Modern, Legacy, and Pioneer) Try to start with a commander that doesn’t seem complicated. Try something like one of the Niv-Mizzets. They’re simple, yet very effective. Then, you want to establish a way to win the game, it doesn’t have to be going infinite, but simply a way that works. You can do 1 of 2 things, you can either win suddenly or win incrementally (slowly throughout the game), both have their pros and cons. A card like Torbran uses red burn spells to win through via damage or red creatures to buff up, both giving incremental damage. While a card like “Tasigur, Golden Fang” wins via combos and ‘out of nowhere’. Combos tend to be less liked in the community, but are still fairly common to come across and widely accepted. You have to analyze your commander and decide what build you want to create and add cards that have synergy with it.
For formats like Pioneer and Legacy, you have to find a strategy that you like. Do you like big creatures > try a big boys deck that relies on fast mana to get out creatures and seal the game (Usually in red / green or mono green). Do you enjoy stopping others plans and to be in control of the game > Maybe try a control build (Usually in white / blue or white / blue / black). If you want to @ me and get a more specific guide or want me to go in more depth, feel free to let me know and I’ll get back as soon as possible. Hope you have fun through your ups and downs and you find your “baby deck” (your deck that you love more than crack itself).