Quick answer
el = the masculine singular article, used before a masculine noun.
lo = the neuter article, used before an adjective or a relative clause to mean "the … thing" or "what".
lo is never followed by a noun. If a noun comes next, you need el or la.
Side by side
el bueno / lo buenoThe good one (a person or thing) versus the good part, the good thing about it.
Lo que necesito es tiempo.What I need is time — lo que is the standard translation of a headless "what".
No sabes lo difícil que fue.You don't know how hard it was — lo plus adjective plus que expresses degree.
Memory trick
Look at the next word. A noun takes el or la; an adjective or a <i>que</i>-clause takes lo.
Typical mistakes
Lo problema es el dinero.
El problema es el dinero.
Problema is a noun, and a masculine one despite the -a ending, so it takes el. Lo never precedes a noun.
El que necesito es más tiempo.
Lo que necesito es más tiempo.
Here "what" refers to no specific noun, so it is lo que. <i>El que</i> would mean "the one who".
Check yourself
Fill in: "___ importante es llegar a tiempo."