can someone help me. i need a place to level u my pokemon to around level 50 in platinum, where is the best place for me to do this?
Fight/Resort/Survival Area, especially the Trainer Cafe in the latter. If you are not there yet, try Victory Road, Route 230, or Lake Valor.
were is the best place to train a lvl 18 grass type?
In DP:
Route 212 South: Wooper lv 18 and 30% encounter rate
Route 212 South: Rangers with 2 Prinplup, 1 Marill, 1 Aipom all lv 21
Route 212 South: Collector with 3 Geodude lv 17-19
Route 212 South: Fisherman with 3 Barboach and 1 Shellos lv 17-20
Route 212 South: Fisherman with Goldeen and Barboach all lv 19
Route 212 South: Parasol Lady with Goldeen and Buizel all lv 19
Route 213: Buizel lv 20-21 and 30% encounter
Route 213: Tuber Boy with 3 Shellos lv 19-21
In Platinum:
Route 207: Hiker with Geodude and Nosepass lv 20
Route 207: Hiker with 3 Geodude and 1 Zubat lv 15-17
Route 208: Hiker with Onix lv 22
Route 210 North: Good Rod Fishing Magikarp and Barboach lv 10-25
Route 210 South: Geodude lv 18 and 30% encounter
Route 211 West: Hiker with lv 14/18 Geodude/Onix
Route 212 North: Marill lv 21-23 and 25% encounter rate
Route 212 South: Shellos lv 23-26 45% encounter rate
Feel free to use
Bulbapedia to find more locations.