What do you check first when a stock's volume suddenly spikes during the day?

I'm still learning, so forgive me if I have the basics off. From what I've read, volume is the number of shares that changed hands in a period. A big jump can mean more people are interested, but I don't know yet how to tell whether it means anything on its own. Someone told me once that it helps to compare today's volume with the usual volume for that stock, not just look at the raw number. I'm not sure what counts as "usual," though, or how long people look back to work that out. What I'd like to understand is the order of checks. Do people look at the time of day, the price move, or whether there was news first? And what do you wish someone had told you about this when you started?