As humans we have a natural curve in the spine which we should respect when playing the cello. This does not mean we should be hunched or slouched but take care not to sit overly straight, relax.
Invest in a special cello cushion which will tilt your pelvis and enable you to gain the ideal 135 degree position. The cushions come in a variety of sizes and will enable you to stay stable and balanced.
Seat height is very important. Chairs which are too low will position your knees above your hips and this can put strain on the lower back. Take advice from fellow cello players and perhaps test their chairs to help you understand the positioning better.
To find an optimal position for the neck/head during cello playing, think about nodding. Find the easiest position for nodding your head and then adopt that during your cello playing. If your neck is sticking out too far it will affect your ability to play.
Feet should always be positioned flat on the floor.