Suppose freshman year, first semester i get a 3.25 gpa, and the last semester of freshman year i get a 4.0. assume i get a 4.0 the rest of my high school career.. what gpa can i end with? and will it be a good gpa ? ?

It depends on how many semesters you have
in my school ther are 2 symesters so
to find the gpa you average

4 years in hight school
2 ysmpersers each
4 times 2=8
so we do
total gpa=3.25+4+4+4+4+4+4+4=31.25
to take the average, we divide by number of symesters
31.25/8=3.9
average gpa is 3.9 which is decent
sometimes you need over 4.0 so take some AP or honors or weighted classes