It is a good idea to carry a small amount of US dollars in cash for emergencies. Major credit cards are now generally accepted throughout the world and are a great way to pay for little luxuries. There is never enough time when you start travelling, there is always that something extra to see. I strongly recommend flexibility as the key to planning. You generally need to know some imminent plans - but the long term can be more flexible.
Most people restrict themselves to the one year duration of their airline tickets.
However if you only have a little time concentrate on a smaller area. Don't try to visit every country that you see on the map, you will end up too worn out to appreciate anything. It is a common feeling to be apprehensive about setting off on the unknown by yourself. However, there can be as many or more drawbacks travelling with someone as going solo. Travelling is a twenty four hour occupation and unless you know each other very well, those small irritations at home can suddenly become a lot worse on the road.