Here you go : https://www.eurail.com/en/get-inspired/top-destinations/switzerland-train/swiss-railway-companies-discounted

50% discount for Rigi or Pilatus

You do not book anything on Rail Planner, you buy through the railway companies themselves. Can be done online but also at the station, tickets cost the same and cannot sell out

Seat reservation for trains in Switzerland is a waste of money. You only need them if you want to travel with the trains/coaches with mandatory reservation on the tourist trains on the scenic routes. However, on these routes, you better use the reservation free coaches or reservation free regional trains. 

Thank you @AnnaB @thibcabe 

If I want to go to Mt Rigi and Pilatus,

which regional train I need to take ? I meant can you guide station names ? 
 

we are staying in Lucerne for 6N . So can you tell me which station will be near to those attractions ?

I recommend you to download the app of SBB (Swiss Railway) and use that for finding your connections. 

There are different ways to reach the Rigi Kulm.

If you only will be travelling in Switzerland, a Swiss Travel Pass, will most likely be better value than a Eurail global pass.

Do you have children travelling with you? With a Swiss Travel Pass, children under 16 travel for free.

Certainly, with your Eurail Global Pass, you can conveniently explore the major attractions in Switzerland from Lucerne. Begin by exploring the beautiful city of Lucerne itself, including its iconic Chapel Bridge, the Lion Monument, and the charming Old Town. To visit Mt. Rigi, take a train from Lucerne to Vitznau and then board the cogwheel train that take you to the summit, where your can enjoy breathtaking views. 
Thanks and Regards

@DamonSantino There are no trains to Vitznau

Either train to Arth-Goldau (covered) + cogwheel train to Rigi (50% discount) OR boat to Vitznau and cogwheel train to Rigi (all 50% discount)

is it better to plan from Lucerne or weggis ? can i take boat from weggies to Vitznau ?is it included in eurorail pass?

I've already answered all your questions.

There is a boat between Weggis and Vitznau (50% discount) or a bus (not included)

You plan from wherever you want or wherever your accomodation is, use the SBB app

Thank you. should I use my travel days in order to have boat rides ? 

No, you don't need to use a pass day for discounts. Same for Vitznau - Rigi Kulm cogwheel train