train

Where are you coming from? Where does your train originate?

It sounds as if you are referring to Eurostar from London. Security check and passport control is done on boarding at St Pancras. In Paris you simply disembark and proceed on your way.
http://www.eurostar.com/uk-en/travel-info/your-trip/epassport-gates

ray,

I assume you're referring to the EuroStar from London to Paris? If that's the case, you'll clear French Passport control at London St. Pancras, so be sure to arrive there early! You MUST check in at least 30 minutes before departure, or you won't be riding.

Some information on where your tour will be staring would also be helpful, as you'll need to allow time to reach that area from Gare du Nord. Were you planning to check-in to your hotel and drop your luggage before the tour?

A bit more information would be very helpful.

That was really quick and thank you all for your prompt reply.
Sorry I did not provide enough details. I am taking the train from London to Paris and will reach Gare du Nord at around 10:20. I have hotel booked at 1st or 2nd paris arrondissement. I am hoping to take a cab and drop off luggage at the hotel and head off for the tour. The departure point for the tour is at 41, Avenue de la Bourdonnais 75007 PARIS, near the Eiffel Tower and starts at 1 pm. will this be enough time for getting to the tour or am I rushing it?
please advise
sincerely
Reema

I think that is a reasonable amount of time. Seems like you have a good chance of being at you hotel by 11. You may not even be able to access your room that early, so they can hold your bags at reception and you can head to your tour, maybe with enough time for a light lunch.

thank you Sam for your prompt reply.
sincerely
Reema

At Gare du Nord, don't go with anyone who approaches you on foot offering a taxi.
Follow the overhead signs to the TAXI stand outside the station.
Remember that legitimate taxi drivers always remain in their vehicles.
Hand the driver the complete printed address of your hotel - including the Postal Code.
That way, you can avoid any confusion. You should have time to make your tour.

After you check in and drop your luggage, ask the hotel desk clerk to call you a cab and give you a map, if you don't have one.

Write down the address where the tour begins and hand it to the driver.
The ride to your hotel should cost approximately 20 EU, depending on your exact address.
The ride to the tour should cost about the same amount.

If you want to take a taxi back to the hotel, walk to a busy intersection and look for a taxi with a green light on top. Stick out your arm and smile, hand the driver the address of your hotel.

You should be able to manage these conservative timings ...

1045 Taxi from Gare du Nord to hotel
1115 Arrive hotel and deposit bags
1145 Depart hotel on foot crossing the Seine to Orsay RER Station
1215 RER train from Orsay to Tour Eiffel/Champ de Mars
1230 Start walking from station to departure point
1245 Arrive tour departure point

thank you all for your replies- greatly appreciated.
sincerely
Reema

http://www.ratp.fr/en/ratp/c_21879/visiting-paris/

This interactive tool can help you plan travel in Paris. It's fairly complicated but it can sketch out routes from one place to another. I suggest you start playing with it now so you will be more comfortable once you arrive in the city.
Google Maps is also getting better at showing mass transit routes.

Gare du Nord can be a bit seedy, sketchy and overwhelming - don't let it overwhelm you.