Even before you get a proper 'bump' you may find your waist thickens, making it hard to fit into your tightest clothes.
You may be able to adapt your clothes for a while - leaving the top button undone on trousers might be enough, and you can hide the gap with a longer top.
Lots of today's maternity clothes can be worn after pregnancy, too, and some styles of normal clothes are well suited to covering growing bumps. You could also borrow some bigger items from friends or family.
It's difficult to predict how big you'll get; even previous pregnancies can only give a rough guide. This makes it hard to buy in advance of actual need. But however large you become, you'll certainly start needing something more 'roomy' from six months. And remember that clothes with the most 'space' will give you the most wear.