sorry I am very simple minded.
If I currently own 1 btc, does it mean that I also own one XT?
Yes, but only after the fork happens, which at its earliest date is 1/11/2016
If I have BTC on exchange I dont own XT.
Unless that exchange is using both XT and Core, which is unlikely
If I will buy 10 btc only next year I will not own XT at all.
Only if the fork has happened. There is no guarantee the fork will happen by next year. But if it does happen and you buy 10 btc, then you probably won't have XT. It is possible for you to also get Xt if the inputs of the transaction for the 10 btc were from before the fork. If that happens, then you could have both XT and btc since the transaction would be valid on both chains.
Also, keep in mind that if the fork occurs with consensus, then it is very unlikely that any major exchanges, miners, or businesses will continue to use Bitcoin Core on the old chain and will then switch to the XT chain which will be called Bitcoin.