Seishun 18 Kippu Tokyo,Kyoto,Hiroshima

Im travelling to Tokyo the 27th of July and have 10 days to see Japan. I was thinking getting the night train (the one you can use with Seishun 18 Kippu ) to Kyoto, stay for one night, travel to Osaka & Kobe spend the night and get the first train to Hiroshima for a day and the night train back to Tokyo (then travel to Yokohama etc). The question is can I use the Seishun 18 Kippu on weekends, do I have to use for 5 days in a row, how do I book tickets and is this a good idea ;) Im on a budget so cant really use the fastest bullet trains to all the places.
Or is it better to get the 7 days train pass? I live in Bangkok at the moment, can I get it from here??
Im very confused and have never been in Japan before.

The night train from Tokyo to Kyoto (actually, it goes only as far as Ogaki) is very popular, requires seat reservations and can get booked out. If you can't get seat reservations, you will have to have a backup plan. Note that it takes 9 hours to get from Tokyo to Kyoto by daytime trains.

get the first train to Hiroshima

It takes six hours to get from Kobe to Hiroshima by local trains. If you take the first train in the morning, you can get to Hiroshima by around 11:30.

for a day and the night train back to Tokyo

There is no such night train! By daytime trains, it takes about 15 hours!

The question is can I use the Seishun 18 Kippu on weekends, do I have to use for 5 days in a row, how do I book tickets

It is all explained on the following page:
http://www.japan-guide.com/e/e2362.html

and is this a good idea ;)

Obviously not :(

Im on a budget so cant really use the fastest bullet trains to all the places.

Then I recommend looking into night buses:
http://www.japan-guide.com/e/e2366.html

Thank you for your answer but do you think you can help me find another way..
You can tak a night train from tokyo to Hiroshima and arrive at 4 on the afternoon so why cant you do the same from Hiroshima?
I still cant any information if you can use the card during weekends..
And do you think its better to get a 7 day pass and will that include faster train?
Thank you for the nightbus tip but I have lived in Thailand long enough to know that they are not safe and I cant stand them anymore ;)

The kippu can be used on any day of the week, the reason there's no mention of weekend is because it is implied that it is valid for any day of the week.

You can take a night train from tokyo to Hiroshima and arrive at 4 on the afternoon so why cant you do the same from Hiroshima?

No you can't take a night train all the way from Tokyo to Hiroshima. As mentioned, like your trip to Kyoto. You take a night train from Tokyo to Ogaki Station, which is in Central Japan near Nagoya, after that you travel an additional 8.5hrs by commuter trains (with 4+ transfers) to get to Hiroshima.

So if you took a day trip from Kobe to Hiroshima, you get into Hiroshima at 11:30. You would have to leave Hiroshima at around 13:00 to make all appropriate connections back to Ogaki. So it would be impossible to have a day trip to Hiroshima.

Also the night train Moonlight Nagara is a hugely popular train, and reservations often sell out very quickly, so as mentioned by Uji it is good to have a back up plan.

As for buses,
I don't know much about Thailand, but I can tell you buses in Japan are probably 1,000% safer and reliable then buses found in Thailand.

As for the Rail Pass.
Yes if you are willing to pay extra the 7 Day version of the Japan rail pass will allow you to take the much faster Shinkansen "bullet trains" which will reduce your travel times very significantly. For example Tokyo-Kyoto direct trains in 2hrs 40 minutes on the Rail Pass vs nearly 9 hours and multiple transfers on the 18 kippu.