Mesos is used to buy good weapons, if you want to upgrade.
When your level is 71, Voodoos and Hoodoos from the Haunted House make the best training choice for not only White Knights, but any other class around this level. As this area is always packed with other maples trying to train, the map is small and enclosed and there is a quick spawn, but the only bad thing is trying to find a free channel. So you can level 82 easily and quickly.
Form level 83 to 100 Dual you should kill Ghost Pirates in Ludicrous. The bad thing about the map here though is that occasionally many hacking Dragon Knights come and steal the map from White Knights. Trying to train and many times the hacker will leave their Dragon Knights there to steal EXP for hours on end, and so sometimes trying to train here can be difficult. You should have enough money to buy powerful equipment.
At Level 101, you will meet Grim Phantom Watches in Ludicrous. Because the thing about this map is that it is very large, the Grim Phantom watches are spread out on multiple platforms, this makes it very time consuming for a White Knight to walk around and kill Grim. But, nonetheless, Grim Phantom Watches provide good EXP. After this, your level will be 110.
You next target is Blue Wyverns in Leafed. The map is small and there is a relatively fast spawn for Wyverns here. Although pot costly, Blue Wyverns make the best solo training choice for White Knights around this level. A bad thing about the map, if you do you will be sent to the previous map though is that there is a pit you can fall into at the bottom of the map.
Training from level 121 really depends on the skills in Maple story that you have obtained in your 4th job. If you have Divine or Holy Charge, when you are done waiting those 4 levels you can go home or you can train with a priest, wait for about 4 levels on it while putting skill points on it. Depending on your strength, you can also try to fight S too using your Divine or Holy Charge.