Hello Cristina, This is one of the most frequently asked questions in meditation. I'm glad you posted it here! Comfort is so essential to meditation, so if you feel uncomfortable in half lotus position you can change your position. Otherwise your body will become a distraction in meditation.

You can sit on a chair, make sure that your back and neck is still straight, you can use back support if you need it....Like Nicky is doing here...

When sitting on a chair make sure it is the correct height, your feet should be flat on the floor, and your knees form a 90 degree angle.

Hope you always feel comfortable when meditating, and experience deep inner peace

Anita Nuss.... i think u are good at this... and its nice that every body else is ready to help every body else here