Businesses you don't understand -- Here's a great rule of thumb that works for beginners and expert investors alike. If you can't clearly explain what a company does and how it makes money in a sentence or two, don't invest in it. There are literally thousands of publicly traded companies to choose from, and you should be able to find plenty of opportunities in easy-to-understand businesses.
In contrast, professional fund managers (information here) do not want tips. They have dozens of good ideas of their own. They won't be sharing those ideas with you and they will not be expecting you to share yours. Instead, they ask about how you allocate money. "Which sectors and markets do you like and why?" The difference between these approaches is like night and day.
Why I’m buying: Spell out what you find attractive about the company and the opportunity you see for the future. What are your expectations? What metrics matter most and what milestones will you use to judge the company’s progress? Catalog the potential pitfalls and mark which ones would be game-changers and which would be signs of a temporary setback.