They are actually all pretty good, and only need some minor changes:
I admit at certain times I am too serious and I should lighten up a bit, but if you don't take my advice now seriously, your life will be in danger.
--> I admit that I am sometimes too serious and should lighten up a bit, but if you don't take my advice seriously now, your life will be in danger.
I realize that if I go around with this grim expression on my face, not even one person won't come up to have a conversation with me.
--> I realize that if I go around with this grim expression on my face, not even one person (or, not a single person) will come up to strike up a conversation with me. (PS We try to avoid double negatives in English ... not even one person .... won't =double negative).
Climbing a precarious ladder can be dangerous.
--> OK! But a little bookish. --> Climbing an unstable ladder can be dangerous.
Climbing a mountain is perilous as non-knowledge can result in death.
--> Climbing a mountain is dangerous, as one false step can result in death. (Or, as inexperience can lead to death).
There are many informational sources around the world, but finding the informative out of all of them is the trickiest.
--> There are many sources of information in the world, but finding the most informative of them all is quite tricky.