Want to Invest on ETH (QUESTION)

how to know when the price will go down? (i want to invest when the price goes down)...

is there any site where i can deposit money then just instantly buy when it goes down to a specific amount?

ex: when Ethereum's price decreases to $2xx, immediately purchase a lot...

If you use poloniex you can try theGUNBOT service

Thats a standard limit order on an exchange. Kraken or Gdax would be the obvious ones for buying with usd etc. Poloniex for buying with btc.