In Death of a Salesman, what mistakes did Willy make at the beginning of the play where his memories flow back to when Biff is in high school?

Willy made many mistakes.  These early memories concern
mostly his parenting mistakes.   Early in the play, when he remembers Biff and Happy as
teenagers, he advises Biff to be "careful with those girls."  He tells Biff not to
promise them anything, to take care of his schooling first.  Yet, he is also  proud of
Biff that girls "pay for you."  This type of ambiguous message is typical of Willy's
parenting.  He contradicts himself often.

His next memory
involves Willy's coming home from a business trip.  Here he ignores Happy and focuses
only on Biff.  We see the effects of Happy being ignored when we look closely at the
shallow, immoral adult Happy has become.  He also praises Biff for stealing a football
for practice.  He calls this "initiative."  Biff later is fired for stealing from
Oliver; and when he goes to Oliver to ask for a stake in a business he and Happy are
trying to start, Biff leaves with a stolen pen from Oliver's desk.  Later we learn that
Biff has been in prison.

Additionally, Willy disparages
Bernard who attempts to tutor Biff in math as someone who is not "well liked."  He tells
Biff and Happy that they are built like Adonises and that the "man who makes an
appearance in the business world . . .is the man who gets ahead."  We later see that
Biff is the one who failed math, never got his high school degree, never went on to
college and that Bernard is the one who became a successful lawyer arguing cases in
front of the Supreme Court.

Other mistakes involve Willy as
a provider and a husband.  In the following  flashback with Linda, we see that Willy
never really was a successful salesman, never really making enough to make ends meet,
that Willy's colleagues laugh at him, that he talks too much, and that he is overweight.
 He is not the successful business man that he leads his sons to believe that he is.
 This reminiscence is interrupted with sounds of a woman's laughter.  We later learn
that this laughter is that of a woman with whom Willy is having an affair.  Biff's
discovery of this affair is what led to Biff's alienation from his father and his
leaving home without a high school degree.

Willy thinks his
biggest mistake was not going with his brother Ben to Alaska, a proposition that Willy
believes would have made him rich.  But this was not truly a mistake.  Willy's true
mistake was not being a better father to his sons or a better husband to his wife.
 Willy's mistake is that he did not face the reality that he was not a financial success
and that Biff was not perfect.