It really depends on you. Your fetus is pretty tiny in the first two months, so other people won't be able to see much-if any-change in your belly. If this is your first pregnancy, it often shows later than women who've already been through it once.
By about 12 weeks, when the top of the uterus has grown up and out of the pelvic cavity, you'll probably be able to feel it just above the pubic bone. This significant change usually signals the beginnings of your visible baby bump. That means it's time to start the maternity clothes shopping!