Look up cat yoga position. Its where you kneel on your knees and put your hand on the floor then you can curv your back upwards.
After my initial stretch I bring one knee half way to my body and put my ankle on the side of my knee closest to my body. I pull into me and hold for 8 seconds. I then twist to each side holding for 8 secs. Then repeat with my other leg. This stretches the outer thigh and my back.
Forward fold, and also stretching your hamstrings can alleviate tension in the back. Stretching your shoulders and cat-cow yoga poses help also.
Try the cat-cow movement – I learned it at yoga. Get on all fours – make sure your knees are directly below your hips, and your wrists directly below your shoulders. Neck straight, looking down. Breathe in, look up at the ceiling, dip your back and tilt your backside upwards. Breathe out, curve your spine, tuck your backside in and look between your knees. Repeat until you feel nice and loose.
For streching your back, thats a huge area of many musclegroups. But you cab start with doing cat/cow (see a yogavideo on that). You can strech lover back by standing and rolling down your head until your hands hit the floor, ben you knees a bit or more! Allways breathe deeply when you strech, that way your body can relax more.