1st as your new you need to pick a formation and stick to it for a season or two!, buy players to suit that formation and keep upgrading your stadium and accessories, dont over train your players so start with once per day! never before a match as not to tire your team before you play!,. You can move your players around!, you just drag a player over the position you want and press save, it is best not to play players out of possession in the long run as you lose your possession bonus during matches, you can get away with some like a DML IN A DL position thats not to bad but just dont try play a ST in a DC although you will probably come across teams with players playing in a stupid position but I advise against it!.
As for changing the game during live games you can do this by going to your formation settings and changing them! this can help or make it worse its a lottery really!, pluss depending what formation you start with you could put on another striker if playing 1 or 2, put on another midfielder if playing 3!, put on take off a defender!! can you see where im going!, no one here can tell you the correct way as every match is different due to many factor of the two teams!, only you can decide what you think is best.