Oooh, oooh, I can be a #finfluencer!
*clears throat*
Buy shares in an index fund or two or three or four. Cover the market as best you can. Automate your investments, whether it be yearly, quarterly, monthly, fortnightly, weekly, or daily. Re-invest your dividends and capital gains. Do not take money out until it is needed. Only move money around if, over the course of a year or more, the index fund underperforms your other index funds. Automating deposits + "buy low, sell high" = it will even out over time.
USA: Remember that investment income is income and must be reported on your taxes; your investment site will provide you with the forms.
If you cannot afford the minimum buy-in to an index fund (Vanguard's tend to be ~$3,000), then you do not have enough spare cash on hand to be investing in anything. Remember that you will keep your shares in the index fund, but the price can crash, leaving you with the same number of shares but less money. Do not take money out until it is needed; the more shares you have when it's low means the more money you have when it's high.
Do not pick stocks. Ever. Even that one. I mean it. If you must pick stocks, have a small child or an animal pick them for you; random chance is better than your educated guess.
Anyone who tells you they can outperform the market and have major successes every quarter is incorrect at best and committing a crime at worst.
