It really depends on the bike. For some, it can be as easy as opening a box and snapping in a few pedals. Others may require more detailed assembly, like attaching the front wheel and handlebars or adjusting the brakes. If you're not confident in your ability to put together a bike, there are plenty of reputable shops that can do it for you. The cost of professional assembly will vary depending on the complexity of the job but is usually fairly reasonable.