My daughter and I are staying in Paris the end of April for a week then heading to London. We are trying to figure out if we should book our train tickets to London now. Any pointers? Also, we are staying near the Louvre, how do we figure out which station to book our tickets at?
Yes, book them now. The longer you wait, the more expensive they will be. The train leaves from Gare du Nord in Paris, which is served by many subway lines. You won't have any problem getting there from your hotel. Book your tickets at www.eurostar.com. If you can be flexible with dates/times, you can get one-way standard for £44/person (adult), or cheaper if your daughter is younger than 26.
At Eurostar.com you can find advance booking tickets for as low as $98 one-way. Those fares will probably be sold out long before you arrive in Paris, so book ASAP (eg, the discount tickets for 11 Feb are long gone). These tickets are non refundable and non changeable. Eurostar trains only run to/from Paris Gare du Nord, and now end up at the new terminal at London St. Pancras.
You can also book these tickets through the railpass section of this website. It's very easy. I was able to get a youth ticket for my daughter for $58.00.