Eli Bendersky's website One of the most common grammatical mistakes (even among Americans) is to mix up its and it's.
So here is the rule: its is the possessive pronoun, meaning of it or belonging to it (As in "The team lost its spirit"). On the other hand, it's is a contraction of it is or it has (As in "It's obvious that he is wrong"). There's a big difference, and the two forms are almost never interchangeable.
Another common mistake is to mix up lets and let's. I'm also prone to this mistake sometimes, and this has to stop !
The rule is: let's is a contraction of let us (As in "Let's play soccer!"). But lets is just the 3rd form singular of to let (As in "He lets us read sometimes"). These aren't interchangeable either, and you should know exactly which one you need to use.