That's a typical posture problem for people who are constantly seated at work, especially when you use computers. Let's break it down:
Problem: Head slouches forward
Cause: Head is usually tilted down towards the desk or leaning forward towards a screen.
Ways to Solve: Unfortunately there aren't too many exercises you can use to correct this. There is, perhaps, the neck extension exercise where you lay on a bench facing down with your head off the end of the bench. Start with your head tilted down, then apply light resistance (probably using a hand) to the back of your head while you lift your head up.
Problem: Rounded shoulders
Cause: Arms constantly out in front of you, cause your chest to be tight and upper back to be loose.
Ways to Solve: Start putting more emphasis on exercising your upper back (rows, deadlifts, etc.) and stretching the chest.
Problem: Excessive arching of the lower back.
Cause: Staying in a seated position for long durations. This causes the hip flexors to become tight and the glutes to weaken, therefore the butt will stick out more when in a standing position with the hip tilting forward. In order for the upper body to remain in an upright position, the body is forced to arch the lower back more, eventually causing excessive stress and tightening of the lower back and weakening of the abdominals.
Ways to Solve: Stretch the hip flexors and exercise the glutes (butt). Make sure to EMPHASIZE the glutes because with tight hip flexors, the body will compensate the limited range of motion by arching the back rather than utilizing the weak glutes. The function of the glutes is to tilt the hip back to either bring your legs or spine back, while the lower back bends the spine back; they usually work in unison but have completely different functions. You also need to work on the abdominals WITHOUT exercising the hip flexors at the same time. The function of the rectus abdominals and obliques is moving the spine (bending forward, sideways, and rotation), while hip flexors bends the body at the hip. This means when you have to refrain from bending your hip when doing abs exercises like crunches (NOTE: also try not to tilt your head towards your chest as well when doing abs exercises, doing so would encourage the head slouching forward problem). You could try keeping the glutes squeezed throughout the movement to help prevent bending at the hip.
This is just to breakdown the issue at hand down to basics to help you understand the root cause. There are plenty of exercises for the most part that you can use to help, pick which ever ones you're more comfortable with. Awareness is the key, though.