The platform is not on the ticket (it never is on train tickets either, is it?), but the signs are easy enough. Work out from the tube map first where you want to go and what line that is on in which direction. Then follow the signs for that, such as "Victoria line Southbound". Be careful taking the Northern line going south as it splits after Euston so you need to know whether you want the Charing Cross branch or the Bank branch. There is a separate platform for each. It's not hard, or all the tourists would get lost!
Do try and avoid buying single tickets unless you're only doing one or two journeys, as this is far more expensive than any other way of doing it. Buy an Oystercard instead and this is much cheaper. This works like a "pay as you go" phone - put money on it and the ticket barriers automatically deduct the right fare. When the money runs out, add more at a ticket machine. You pay an extra ￡5 for the card but this is refundable if you give the card back. This is cheaper than buying a one day Travelcard as those now cost ￡12, much more than the daily cap on Oyster - when you hit the daily cap, which is ￡6.40 if you only travel in Zone 1, you go free for the rest of the day.
To use an Oystercard, touch it on the round yellow reader when you go through a ticket barrier, both when you enter the station and when you leave at the other end, and it does the rest for you. Oyster also works on buses. Touch the reader when you get on and the ￡1.50 fare is knocked off. London buses no longer take cash so now you know how not to look an idiot if you want to ride on one!
Or (and this is TfL's latest thing), if you have a contactless debit or credit card, you don't need to buy a ticket at all. Touch your card to the yellow reader just the same as Oyster, and it works the same way. You also get the cheaper Oyster fare, not the cash fare.
There are big square ticket machines inside the tube stations that sell tube tickets (similar to the ones that sell regular train tickets, but they are different machines), as well as a manned ticket desk, you cant miss them becasue you have to go past them to get to any of the platforms, and the barriers wont let you in without a ticket anyways. If your buying a through-ticket (as in, to take another train from within London), you might also end up with a travelcard for the tube included with your train fare, in which case you just scan the same ticket.
The platform wont be indicated on the tube ticket, but there is clear and easy to interpret signage pointing to which platform is which line, and there are plenty of maps in the stations so you can plan your route before you get on. It's really self-explanitory when you get there. I was able to understand it from the moment I got to london, having never used an underground before in my life.
Also, a bit of advice from me. If your planning on using the tube multiple times that day, can I advise you buy an Oyster card rather than a ticket (some machines dispense them, and you can buy from the ticket office). An all day travelcard costs 12 pounds, an Oyster costs 5 pounds and is capped at a maximum of 7 pounds a day (after which travel is free). When you no longer need the card, you can turn it in at the station for your 5 pounds back, along with any unused credit.
This plan of London Euston Station shows you where to buy tickets and etc.
http://www.nationalrail.co.uk/SME/html/N...
If you want further help at London Euston - go to the Information Desk which you an see on the above plan - a white oblong - next to platform 8
https://www.youtube.com/watch?v=9QF5vTNr...
Within Euston station you look for the UNDERGROUND sign. Once you've gone down the escalator into the tube station, you'll find the ticket machines.
You can buy them from the ticket machine at the entrance to the Tube station, just like EVERYWHERE ELSE in London.
