You got to feed it a bunch (tip use ure berry pot if ure low on them) and when it fully healed you get some milk tank milk and a tm I believe and you can also buy it then
What Happens When You Give The Oran Berries To The Sick Miltank?
When you give it 7 Oran Berries the girl will give you the Seal Case.
You give the Miltank about... Six berries I think, and it feels better so the farmer gives you Moo-Moo Milk (healing item) and yes, a TM... Rollout I think