You should probably do more homework if you need to ask these types of questions, but to answer it anyways, Netflix (NFLX) is doing well, but not as well as Amazon (AMZN), Tesla(TSLA) has been doing very well, Google (GOOG, and GOOGL) are also doing well. However, stocks can only do well for so long...its anyones guess as to if these stocks are on a roll, and keep going up, or if they've reached their peak and will soon start coming down. As you might have noticed, I focus mainly on Tech stocks, but overall, esp if you are just beginning, you should look more into diversification, such as different mutual funds.
As for if you can only invest in it once, If you have $100, and you spend $50 for 1 share of stock of something, you'll have $50 left, and 1 share of something. You can either buy something else with the other $50 you have, or sell the 1 share of something, or both. If you sell the 1 share of something for $60, you'll have $110. You can then take your $110 and buy something else, or the same stock you just sold. (and vice versa, if you sell your 1 share of something for $40, then you'll have a total of $90 total left to buy something else.)