There are several great yoga positions for beginners. Theses postures are great to begin because they will allow you to learn some basic guidelines of yoga, without necessarily being too much of a challenge for your body. So start with beginner poses and work your way toward more difficult ones progressively.
If your fitness level is already very good, it does not mean that beginner yoga poses are not for you. Advanced yogis practice these poses too.
Try picking a variety of poses (seated, standing, balancing...) that you think will not be beyond your level of fitness. Begin with poses that will warm-up your spine, such as the Cat Stretch and Cow Pose and Seated Cross Legged Twist . Warm-up your muscles progressively. This is important, like in most physical activity. Warm muscles are less likely to be strained and get injured. They are also more flexible, which means that your range of motion increases.
Postures that target the core area can be included during the warm-up. Working out the core muscles creates a lot of heat within the body and helps the whole body to warm-up. Boat Pose is a great posture that works on the abdominal muscles. Like most poses, it can be modified according to your strength.
As you warm-up, work your way up toward deeper postures. Do not rush through or try to get into a pose too soon. Cold muscles suddenly and roughly stretched out are prone to injury.
The postures below are great yoga positions for beginners to start with. But you can practice them for years and keep getting the benefits out of them. I believe that there is no such thing as "growing out" of a pose. Though you will find them easier and more comfortable to do as you progress into your practice.