I guess you need at least 3 ADA to start staking. When you delegate first time 2 ADA go for transaction fee and 1 ADA left  for pool. Any further redelegation requires as little as 0.17 ADA for fee but the first delegation requires full 2 ADA for in the course of this action  the special  address (which is hidden for you and  relevant to ADA delegated from your wallet) is created.