When you visit a blood bank, you will be asked to fill a form with all your details, including weight, history of previous donation, major and minor illnesses, date of last donation etc.
If you weigh below 45kg, you will not be accepted as a potential donor.
If you have donated blood in the past three months, you will be asked to come back later.
If you are a menstruating female, you can donate, provided you feel comfortable and your haemoglobin level is above 12g/dL; but if you have delivered or if you are lactating, it is not advisable to donate blood for at least six months.
Some amount of blood will be taken for tests like HIV, hepatitis B, hepatitis C, malaria etc.(Please do not bother; this does not mean that you have these diseases or you will get them!