Make reservations either on eurail.com (2€ fee per person per train) or on https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish (4€ fee per order). Alternatively you could also call SNCF to avoid fees altogether.

You could leave it to the last-minute and make them while in Paris but it's risky : first the 10€ limited fare will probably have sold out and the entire train could be sold out.

RER and Metro in Paris are not included in Eurail. On the other hand TER regional trains are.

Before buying a eurail pass you should look at regular fares, it is very possible to get cheaper options for your journey than using Eurail.

https://www.sncf-connect.com/

The shortest journeys CDG - Grenoble do not involve travelling via Paris at all.

@thibcabe  RER and Metro in Paris are not included in Eurail. So is it free if we have Eurail day pass? 

@Al_G Train fare are dynamic with respect time of travel in SNCF. 

Also I would like to roam around paris, before reaching Grenoble. So i have to take multiple trains(RER or Metro), if its free ride with Eurail pass.
Please correct me if im wrong :) 

No it's not that RER and Metros are free with Eurail, it's that you cannot board them with Eurail. You'll have to buy separate tickets.

In that sense regular Paris - Grenoble will probably be best value.

@thibcabe Booking in SNCF in advance is cheaper or buying ticket at train stations(Example, at Paris CDG Aero port to Champ de Mars Eiffel Tower) is cheaper ?

Public transportation in Paris is run by RATP.

I'm no expert but AFAIK you cannot buy public transportation tickets online, use the ticket machines when you get there. You'll probably need a Pass Navigo.