How can I explain this quote by Emerson "life is not intellectual or critical, but sturdy". how can i use examples and what could it be a good topic...

What he's
trying to say here is that you shouldn't spend a lot of time trying to think about stuff or
whine about stuff, but rather take everything as it comes, deal with what life gives you.  In
this same passage, he speaks well of people "who can enjoy what they find, without
questioning."

As far as examples go... it's easier for me to think about
examples of not whining or criticizing.  I bet you can think of things in your own experience
where you can see that it's better to just deal with stuff than to waste energy
complaining.

As for not thinking, the only thing I can think (LOL) of is the
idea that you can paralyze yourself by overthinking things rather than just going with what you
feel.

Good topic sentence?  Depends on what your essay is about.  I'd say
something like "People often make themselves unhappy by thinking too much and worrying too
much."