how do i invest in stocks online?

right now i have about 400 $ saved up and im looking into investing into the stock market. I am looking into long term investments in a few companies however I don't know how to start. i have been looking online at how to trade and how to invest. i have been looking at sites like questrade and virtual broker, my only issue is i don't fully understand the costs involved with making an account. for example virtual broker on their kick program has a " $100 monthly minimum contribution" part. I don't understand if it requires a 100$ more added every month or to just have at least that much in the account for ever. i also do not fully understand the costs involved with buying and selling stock. Im also looking into potentially day trading when i have some free time over a weekend. any insight whether it is a linked article, or book would be well appreciated. Thank you Sincerely NSK