It is generally recommended that babies reach at least 3 months before you can take them outside. Even though babies can be taken outside as early as the first week after being born, their immune system is still not mature and quite weak.
Germs, bacterias and all sort of dust can attack your baby in its first week of being born. Some babies develope asthma due to inhaling heavy polluted dust and being exposed to pollution from the vehicles outside. There are also the risk of someone coughing or sneezing nearby which can cause sickness in babies.
Weather is another factor which will weigh down on your baby's health. Too hot or too cold can cause your baby to quickly get sick, as a baby's body isn't as quick enough to adapt to their outside environment. There is also the danger of baby putting their fingers into their mouth which will allow germs and other microorganisms a good chance of entering their little body.
One rule of thumb to figure out how much clothes babies should wear, is to put one extra layer of clothes as compared to an adult. For example, if you are wearing one long sleeve shirt to go outside, your baby would need to wear two long sleeve shirt to be warm enough.
Be aware of family members or relatives wanting to hold your baby. Try to remind them to wash their hands first before holding baby. Keep in mind, baby's immune system is weak and not fully developed yet. So any introduction of disease or virus will shock the baby's immune system. There have been incidents wherein the baby died because someone in the family had some disease and he kissed the baby on the lips which passed on the disease to the baby. The baby was dead in about a week's time. So be very careful when handling with your baby. I know it is easy to be carried away when so many people wants to hold your cutey pie baby but always remember to be cautious.
It is a good idea to take your baby out for a stroll around the block because it helps the baby build a strong immune system. Babies needs sunlight and fresh air too just like us adults. Studies have shown that kids who are mostly outdoors were able to build a stronger immune system than those kids who stays at home most of the time. This means that the outdoor kids fell sick less often and the indoor kids more often falls sick.
Another tip, is to always wash yourself when you come home and also give your baby a bath to wash away any germs that may have been in contact with the baby's skin. Put all the baby's clothes and blankets which had been outside, for a wash even if you weren't out for long. This is to prevent any germs from surviving on the baby's clothes and blankets.
If you need to treat your guests to celebrate your baby's arrival into the world, at least wait till the baby is 1 year old. Why this is important, is because by that time your baby's immune system is strong enough to tackle any microorganisms and germs it may encounter. It goes without saying, that many guests will be touching, kissing and holding your baby during the celebration and baby will be fine by that age.