Depending on the length of your stay in Swiss territory, you can buy the 3-day, 4-day or 8-day Pass. 2nd class is usually enough.
The Swiss Travel Pass provides you 25% discount from Lauterbrunnen or Grindelwald to Jungfrau (via Klein Scheidegg). Train travel to Lauterbrunnen or Grindelwald (from Interlaken OST) is free with the Swiss Travel Pass and so are your other train travels. Mt. Titlis would have 50% discount with the Swiss Travel Pass.
Below is a detail pricing for Jungfrau with the Swiss Travel Pass :
You can travel via 3 possible routes (prices are per person) :
1 (a). Interlaken OST to Grindelwald (free).
1 (b) Round trip : Gridelwald - Jungfrau (via Klein Scheidegg) - Grindelwald
Cost (w/o discount) : CHF 184
Cost (w/ discount) : CHF 138
Time taken : 1 h 35 m each way.
2 (a). Interlaken OST to Lauterbrunnen (free till Wengen which is after Launterbrunnen but you need to change at Lauterbrunnen and pay only from Wengen to the top)
2 (b) Round trip : Wengen - Jungfrau (via Klein Scheidegg) - Wengen
Cost (w/o discount) : CHF 170
Cost (w/ discount) : CHF 128
Time taken : 1 h 28 m each way
3 (a) Interlaken OST to either of above two places and return via the other way
3 (b) Gridelwald - Jungfrau (via Klein Scheidegg) - Wengen OR the reverse
The cost for this option is between 1 (b) and 2 (b) above. About CHF 10 more than 2 (b). Whichever route you want to travel, you can tell the same at the ticketing counter in Interlaken OST when purchasing your discounted tickets. For further details you can check on this official site (shop.jungfrau.ch/en/ tickets/bergbahn). This site has a host of other options along with weather details at the Jungfrau, skiing options or other activities etc.
Since discounted prices are same whether purchased online or at station, its better to buy at station to avoid any problems or wrong bookings, though during May-June, expect to have a queue at stations.
Also note that the prices I mentioned above are the ticket prices and does not include reservation charges. Reservation might be required during peak seasons.