Read the Help screens. They are great and will answer many of your questions.
Plan your Player Personnel budget wisely. You will need to set aside a certain amount for draftee salaries, international player salaries, and minor league promotions. You also need to have additional money targeted for signing your own FA's and arbitration eligible players. Then you need to decide how much more to set aside for signing open FA's and for WW pickups. A smart GM will also build a cushion on to that. So while it may initially look like you have a ton of cash to spend, it goes quickly.
Look at every column in the player profile!! Sure you may be able to get that stud hitter with the 79 Contact - 87 Power for next to nothing, but look closer. He may turn out to be Milton Bradley. All that power won't do you much good if he gets suspended. Is he worth the risk?