Above attached is the link to book tickets. Absolutely visit the Taj Mahal first thing in the morning. Shah Jahan (One of the Mughal Emporers) built the Taj Mahal in memory of his wife, Mumtaz Mahal. The Taj Mahal is famous as the symbol of love with beautiful architectural details, and studded Jewels. It also has functional architectural details for the monument to last long.
Above is the website to book tickets - I recommend going early in the morning, but if you only have one day to cover the Taj Mahal and Agra Fort, visit the Taj Mahal first.