Hard to get good pictures with Canon EOS600D

I've got a Canon EOS600D. How do I get sharp pictures? My wife always said why is the
background blur. She likes the background and foregrund sharp too. I myself is having a
difficult time choosing the mode. Manual I need some time to adjust if I choose P, AV etc
the picture sometimes is not sharp and the person is very dark but the background is bright.
My wife doesn't like flash. Any advice. Using Tamron 18-270PZD lens. I use this lens because
I don't need to change lens especially travelling. I still got the kit lens. Please advise. Thanks