These should look fresh and bright with no brown patches or black spots. Red chillies tend to be milder than green (because they sweeten as they ripen), and larger ones milder than small. Small red or green Thai chillies, for instance, are especially pungent and strong.
To prepare fresh chillies, first rinse them in cold water. Then, using a small sharp knife, slit them lengthways. For most uses, remove and discard the seeds. Rinse the chillies well under cold running water, and then prepare them according to the recipe’s instructions. Wash your hands, knife and chopping board before preparing other foods, and be very careful not to touch your eyes until you have washed your hands thoroughly with soap and water.