As long as your spine is reasonably straight, it doesn't really matter where or how you sit.

As for thinking... It happens to the most seasoned meditators. In fact, in some respects that's the whole point! It's really helpful to just notice how much our minds shift about. How thoughts intrude our consciousness constantly, even when we think we're being present. Just watch your mind - I find that if I can do it with an attitude of mild amusement it staves away frustration - just laugh at my 'monkey mind', notice what I'm doing and return to the breath. This act of observing self and calmly returning to the object of focus is one of the major benefits - teaches us self-awareness, calm acceptance and an increased ability to return to what we want to focus on in spite of our monkey minds.

I was told to practice breath counting to 10 when I started, but would usually lose count and get frustrated before I got there. So I stuck to 5 instead. Still lost it sometimes but if I could get to 5 a few times without too much interference, I knew I'd made an effort for the day.

It's worth it... persist and you will reap the benefits!