You'll want to take the National Express Coach which will bring you to the port town of Dover in Southern England. The coaches are pretty nice and it's a great way to meet people while traveling through Europe.
While on the coach you'll probably be sleeping most of the time. But if you have some awake time make sure you take in the English countryside in the county of Kent. Mainly rolling hills and farms, but there are many interesting sites along the way.
When you arrive in Dover you'll notice a big castle up on the hill overlooking the rest of the town. Your coach will head down towards the port which will give you great views of the town. Once at the port you'll have to buy a ticket to hop on a ferry to go across the English Channel.
Although there are a few different ferry companies, SeaFrance is a good one to use. Typically you just buy a ticket from the ferry company with the next leaving ferry. Also while on board the ferry you should hit up the bar and turn your little trip into a booze cruise. I know from experience, go with the Kronenberg 1664!
As you leave port on the ferry head towards the back of the ferry to see the White Cliffs of Dover. You'll see awesome views of the Southern Coast of England and you make your way across the English Channel.
When the ferry ride is over you'll arrive in Calais. Not that great of a town, mainly a shipping port, but a good hub to get to many places in Europe. Make your way to the Calais Train Station. I took a taxi which didn't cost much but there are shuttles that can bring you there as well.
You'll probably want to hop on the Eurostar to get you into Paris. There are other train but most of them stop in a lot of towns along the way and can take longer to get to Paris. Make sure you check the time of the train correctly, I may have ended up in Brussels once by having a little miscommunication with the ticket salesman.