Starting to invest

Hi everyone, i'm new to this world and i would love some help 🙂

• My plan is to choose an ETF, invest X amount of money monthly, reinvest dividends passively into that, then wait for 10-20 years

• What i need is any advise you can tell me:
  Any tools that consider money change, or the amount of capital expected by this strategy?
  Example: i thought of investing into SPY (S&P 500) but i didn't consider yet the double taxes (i love in Italy), nor the amount i will receive monthly at the end

Would you kindly introduce me to this world? Thank you all very much