how to write select query ?

i HAVE 2 table PROJ AND VBRP.

I have to display field POST1 from table PROJ.

from table VBRP i have to select field VBELN and PS_PSP_PNR then i have to pass PS_PSP_PNR.

then i have to pass PS_PSP_PNR to table PROJ and select field post1 from proj and display POST1.
HERE i have to use conversion exit for field ps_psp_pnr.