1) Company Names/Brands—You can use company and brand names, but you need to ensure you aren’t portraying that company or brand in a negative light. If in any doubt, it’s better create names and brands of your own.
2) Song Lyrics—This is the big one on which I often have to issue authors with warnings. Song lyrics are copyrighted material. You need to obtain permission (usually paying a fee) from the copyright owner before you can quote them in your novel. Therefore, it is best simply not to use them. State that your character is listening to a certain song by all means, but don’t have him think or say the lyrics.
3) Literary Quotations—If the work in question is in the public domain, you are usually okay. However, in all other cases, you will need to request permission from the copyright owner, who may charge a fee. Poetry is particularly tricky in this regard, since a few lines can be a large percentage of the work. Best to play it safe and leave out those quotes, unless you are certain you have permission, or the work is old enough it doesn’t require it. (N.B. Bear in mind that even if the original work is old, if it’s translated, that translation may be copyrighted in its own right.)