Firstly, welcome to the game and to the forum.
There are many different topics covering the questions that you ask in the forum. But the main thing I would say is do not do anything till the first day of the new season which will be this coming Sunday! Because the players that you have will lose 20% of their ability equivalent to 1 star.
Secondly, don't rush to buy just any old player, there are many tips and tricks on looking for the best players and developing those that you have. Plus, the first 2-3 days of the new season is madness in auctions.
Do not rush to develop your stadium as it is relative to your level.
My Mod colleague Milpol has a hugely successful YouTube channel with tips for new players. I'll point him in your direction to advise you of links etc.
I'm sure others will advise you but as long as you don't rush into anything and do lots of reading you'll be fine.
Enjoy the game and good luck!