Japan Rail Pass

Is buying the Japan Rail pass worth it if I only plan to go to Kyoto from Tokyo or shall i just buy a return ticket?

Regular shinkansen round trip tickets bewteen Tokyo and Kyoto cost about the same as a 7-day Japan Rail Pass:
http://www.japan-guide.com/e/e2363.html

Is buying the Japan Rail pass worth it if I only plan to go to Kyoto from Tokyo or shall i just buy a return ticket?

I'd recommend you to buy point-to-point tickets, because a JR pass doesn't allow you to board the fastest and most frequent Nozomi train.
Nozomi takes 141 minutes between Tokyo and Kyoto and runs 4 to 8 times hourly, while Hikari takes 161 minutes on that route and runs only twice hourly.

In order to save some money, you could queue up for the unreserved seat on the Nozomi at Tokyo Station for 20 minutes to get a seat when you leave for Kyoto, because nearly all the west-bound trains originate at Tokyo.

But you might reserved a seat when you return to Tokyo. Kyoto is an interim stop for all the trains.

Tokyo - Kyoto Nozomi reserved seat: 13,520 yen
Tokyo - Kyoto Nozomi unreserved seat: 12,710 yen
Total: 26,230 yen vs. 28,300 yen of JR pass

Also note that a JR pass is nearly useless to get around in Kyoto. You might buy the pass only if you make a side trip to Hiroshima etc.

In case you stay less than six nights in Kyoto, don't forget that the Japan Rail Pass also covers JR trains in the Tokyo area, including the Narita Express to/from Narita Airport. In Kyoto, it is useful for side trips to Nara, Osaka, Kobe and Himeji.

I didn't recommended OP to buy a JR pass because of the fact that OP stated he or she would make a simple round trip between Tokyo and Kyoto, and because of the following drawbacks of the pass.

1) No admittance into Nozomi: Nozomi runs 8 times between 9 and 10 am on a Tokyo departure basis, while Hikari runs only twice in that time range. Adding that, Hikari takes 20 minutes more. These days reserved seats on Hikari sell out faster than Nozomi's especially on weekends because Jipangu Club members and JR pass holders are forced to use Hikari and Kodama, and due to Hikari Hayawari sales.

2) Tourists must bother to purchase a JR pass voucher before entering Japan. Not all people can buy it at a nearby designated establishment.

3) Pass holders may feel obliged to travel more unnecessarily by train for the sake of the pass.

4) JR pass is nearly useless in Kyoto City proper.

As for Narita Airport access, transport in Tokyo and in Kansai area, there are cheapish means of travels, though OP didn't mentione these matters.

-Suica and Nex package for 3,500 yen, with 1,500 yen suica credit and 500 yen suica deposit
-JR Tokyo 23-Ward unlimited travel one day ticket for 730 yen
-Kyoto-Osaka return for 780 yen by Hankyu
-Kyoto-Himeji return for 2,000 yen on a JR West Kansai Pass, on which one can visit Kobe en route as well. The new rapid service is very frequent and fast enough.

One needs to pay a couple of thousand yen more if he or she chooses to buy point-to-point tickets, but it can be very well compensated for by a Nozomi ride.

though OP didn't mention these matters.