There are no rules when to start having sex again after you have had a baby. However, women may feel sore and tired after childbirth so it's best not to rush it. Make sure you both feel comfortable and you both want it. If it hurts, it won't be pleasurable. You may wish to use some lubricant (available at your pharmacy and large supermarkets) for the first time or first few times as your vagina may feel different because of hormonal changes.
It may be best if the woman explores with her fingers first whether she's ready or not.
Once you're ready make sure you are using contraception. You can fall pregnant again shortly after giving birth. So if you would not like a very small age gap between your kids, use contraception :) Talk to your GP about your options. When choosing the pill when you are breastfeeding, let your GP know that you are breastfeeding and they can give you a pill that won't interfere with breastfeeding.