First, you catch a Croagunk at the Great Marsh in Pastoria City. Then, level your Croagunk up to level 37. Then, your Croagunk should evolve.
How Do You Get Toxicroak The Pokemon On Pokemon Diamond?
Catch a croagunk in the safari zone and get it to lv. 37. After you have national dex, you can catch toxicroak on random days in the safari zone.