When constructing a team the most important factors you need to take into consideration is making sure that you find a scheme and playbook that you are comfortable with and select a Coach and Captain that fits your playstlye. After that it is a matter of surrounding them with players that fit the mould you have created, this is very important as you will get more Team Power and bonuses such as a speedier Overdrive meter.
You will not be able to train individual players like previous Madden NFL Mobile games to get their overall up, instead training a player will raise your team power which will give you an advantage in Overdrive if it's at least 100 more than your opponent. Once you reaching level 5 you will get access to the AH (Auction house) where you will have the ability to buy and sell players to improve your team.
Once you feel you have assembled a decent team try and play as many Overdrive matches as you can to earn Blitz Tokens which you can exchange for packs and hopefully better players.