Walk in through the main entrance at Great Russell Street to the glass covered Great Court and spend a moment marvelling at the wonderful view. It's even better at night, with the court empty of people and the night sky beyond the glass.
This majestic place can't be seen in just one day, so try taking a highlights audio tour, which will take you around the whole museum in a bit more than an hour. It costs $3.50 when I did it, and was enjoyable even if some of the artifacts were hard to find. After you're done, you can go back to revisit the spots that fascinated you most.
Their website is chock full of essential information. It's a great planning tool before you go. Peruse the site here.