I'm going a bit thin on top. This sounds like you have the common 'male pattern baldness' that affects many men any time from puberty to middle age.
Hair also tends to recede from the temples.
It's caused when over-sensitivity to the male hormone testosterone switches on the hair loss gene. Hair loss can also occur because of a lack of iron, an under-active thyroid gland, and scalp infection, so ask your doctor to check these are not the cause for you. Some medicines can cause hair loss, so if you're taking any, have a look on the information sheet to see if this might be the case. The appearance of a fuller head of hair can be achieved by keeping your hair clean and by having an appropriate hairstyle, which a hair stylist will advise you about. For male pattern hair loss at the crown, some men benefit from using the drug minoxidil that is available from the pharmacist.