Question

what if I cross Tt(tall pea plant) with tt(dwarf pea plant,)

Answer 1

you get-
Tt-tall(hybrid)
Tt-tall(hybrid)
tt-dwarf
tt-dwarf

Answer 2

In first F1 generation if all Tt(tall) crossed with tt(dwarf) the new plant occurs to be tall and dwarf in 3 : 1 ratio.