Grab the handlebars, grip the brakes hard, and rock the bike fore and aft just a little. If your headset is loose, you will feel a subtle clunking. Unless you're a pro, let a bike mechanic tighten it. The job requires special tools and a learned touch. A tune-up will also start you off on the right path toward regular maintenance.
Chains are the big issue here.
To protect your engine car shop gurus say change your oil once every 3,000 miles. So we just watch our odometers. A road bike chain that is clean and squeak-free only needs to be replaced every 2,000 miles, says Trainer. A mountain bike chain should be changed once per year, and more frequently if the bike is ridden regularly on muddy trails. Like a car that needs an engine overhaul because its oil has never been changed, a bike with an over-used chain may require a whole new drive train.