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When starting up career mode you will have three game modes inside of career mode itself for you to choose from, "player mode", "manager mode", and "player-manager mode". If you select the traditional "manager mode" then you will be promptly whisked away to select your team.
Selecting your team is one of the most important things in career mode. Whether you choose Manchester City for it's +150,000,000 spending budget or a League 2 side with the hopes of promotion this step is essential.
After selecting the team I generally like to go and check out my new squad. I assess where the problem areas are and where my strong points are where I want to build my team around. So let's say you have a 73 CAM in a team where the average rating is 66. You would want to build your team around him, but be careful, don't put him in a formation where there is 1 or more of his positions, otherwise he might not be as potent. Also, play young guys. This is an extremely important part when selecting your starting XI, I've managed teams where I've had a 17 year old striker with a rating of 65 go all the way to 69 in one season.
Be smart with transfers, they are one of 2 ways to add players to your squad, buying players through the transfer market is one. The first thing I like to do is sell all the players with the lowest ratings. Or players that I deem unworthy. Don't hold on to a young player with a rating of 45 in hopes that he will be the next Messi, clubs will buy, trust me. I then will toggle between the highest and lowest transfer costs and sell accordingly. If I can sell a 70 for $2,000,000 and then buy a player with an equal or greater overall I will. And if I have an aging player with very little stamina I will sell them. (Yes I have sold Del Piero before). But be careful not to sell too many players, if you are competing in domestic and European competitions as well as league then it is important to have good depth within ones squad.
Another thing I do when I start any season is look at the free agents and loanees. Buying free agents and loanees is a great way to save money while still getting some quality players. The best part about buying either of these two is that it won't cost you a single cent of your transfer funds. The only thing that will be depleted is your wage funds, but generally since free agents are not on a team they will join you for a reasonably low price.
On the other hand don't forget to sell young players via the loan option as well. But sell them to clubs you know they will get playing time at, otherwise they might not progress like you want them to.
Simming vs. Playing
Simming games is a great tool when you use it correctly. Use it against teams that you know you could easily beat manually. Don't use it against teams that are close to you in the standings. But be extremely careful when simming away from your stadium, home teams in FIFA automatically have an advantage when simming, for the obvious reasons of course.
One of the best additions to the game this year is the scouting system. Not only is it better than last year's re-generation system it is also extremely fun and is a way to keep you interested. Through scouting I have literally found the next Messi. (A 16 year old from Mexico)
Be careful when selecting your scout though. Select someone that you can afford but is also not total rubbish - the higher the scout the better the players. Also, when sending your scout to the various parts of the world be aware that some regions can produce better players than other regions. For example, if you send a scout to Asia in search of a center back you probably won't find the next Barcelona star. But, if you look in Italy for a playmaker you can probably find some bright prospects.
Selecting Players For the Youth Squad
When selecting players for your youth squad don't be too antsy. But don't be too slow. If you buy them early due to the odd chance they could have an overall rating of 62 at 14 years old you're likely in for a bust. Every month your scout will give you an updated list of new or old players. But if you wait too long other clubs will sign them and they'll end up in the abyss of world football.
End Of the Season
Upon completion of the season you will be able to either stay with your club or move on. When deciding where to go choose the club that you think you can be the most successful with. It's easy right? Not really, sometimes the clubs that have $150,000,000 spending budgets are just too much to resist. One time, due to some rich investors FC Dallas, (an MLS side), had over $100,000,000 dollars to spend. I chose them and ran rampant in the MLS. Winning the championship and the domestic cup 3 years in a row.
Thanks for reading, have fun!