With a date in A1, the formula below will compute the date of the following
Monday
=A1+(WEEKDAY(A1)>1)*(9-WEEKDAY(A1))+(WEEKDAY(A1)=1)
Remember to format as date
Replacing A1 by TODAY() we get
=TODAY()+(WEEKDAY(TODAY())>1)*(9-WEEKDAY(TODAY()))+(WEEKDAY(TODAY())=1)
best wishes
What day is the first day of the following week... Sunday or Monday? Also,
what do you want to show if the date IS the first day of the week... itself
or the first day of the next week?