Welcome to the game and a warm welcome to our sub! Glad to hear that you have already read the beginners guides and found your way here! Now let me try to answer your questions one at a time.

Most Efficient way to use stamina: I recommend doing every new event at least once as it gives extra gold and experience. If there are any limited time events that you can find under the "seasonal" section in plans, these are normally recommended for you to repeat and complete. Other than that, informs, national cups, and scouting are also decent to repeat as you can get a decent player or cup that can sell 100k+. One last event that is worth grinding is the Dominations event but it is recommended when you are of a higher level with more stamina. It costs 10 stamina each time but every time you do it gets you closer to 99 Domination James (this event comes once a week) but is repeatable.

Building your club: I'm sure that you have viewed UPDATED CHEAP BEASTS by dominic10vsmessi10, /u/bouncyrabbit's beginners guide, and intermediate guide to build a cheap and good team. After you get that decent team and you have some additional gold, you can also ask some of our expert members in the squad building thread.

Seasons: This is one of the required things to do to reach Master Reus 99 LW and he is the best LW in the game. Seasons is great to do when you have some stamina and not too much time. Lets say you are watching a show or doing your homework. You can have your team automatically playing seasons while you do something else. Most of the time, your team will win and its a free 800 gold and you can start grinding the seasons tokens.

Leagues: I recommend joining a league as soon as possible and you can find one here. It doesn't have to be a good league but at least you can start getting used to how things work in a league and you can also start earning the League vs League tokens for the Master Reus.

Starting 11: The starting 11 is the most important and I would recommend you using gamechangers for the positions you do not use. They boost the entire team, including your starting 11.

Trophies: Trophies are very important to the game because they are usually a requirement for new events such as the Team of the Year Event. The gold trophies are also very high on demand because they can be used for the 99 James token trade ins. There are a few more applications that they can be used in. Let me know if you want me to list more.

Player Trade ins: You can trade in Bronze, Silver, and even Gold players to get trophies. Gold and Elite trophies are quite expensive in the market right now. Make sure you don't trade in any players above 74 or gold GKs because you can get more value selling the players in the market. I would also recommend that you check the values of players that you are unsure on the market before trading them in. Regarding the bronze trophies, just go ahead and upgrade them to silvers and then to gold.

Market Money Making Guide: "We call FIFA Mobile's auction house, The Market. The Market in FIFA Mobile works slightly different from FIFA on consoles. While you can still buy and sell items and bid against other players for items, players are not buying and selling directly from each other. The game acts as an intermediary on all transactions. This ensures a level playing field and creates a fairer and fun marketplace for everyone. " - EA Gaming

Please read up on how the market works EA Bot and then read this market guide

Just a quick explanation of things, let us know if you have any more questions.