If you are feeling a huge amount of vibration on the headset when you are braking, your headset is loose. This can make the bike difficult to control and dangerous. The frame can also be damaged beyond repair. To tighten it, loosen the top locknut first and then tighten the bottom one. Check it by lifting the top wheel off of the ground. The handlebars should be able to turn smoothly and easily. Adjust the bottom locknut to the correct position and tighten the top locknut against it. If it keeps coming loose your headset is worn.
A dirty stem will cause your bike to creak when pressure is applied to the front end. Solve this by removing the stem and checking that it is greased, clean and not rusty. Remove any grit from the bars by undoing the clamp.
If there are cracks going around the bars where they are clamped to the stems, the handlebars must be replaced.
Leaks are the most common problem with air and oil suspension forks. If the fork is sluggish or there is a slight hissing when they are compressed, it is a sign that there is a leak. You can tell if there is an oil leak if there is oil trickling down the sides . If you have leaks, take the forks to be repaired at a service center.
Common problems with elastomer forks is that they seize up, causing them to be unable to move at all, or they feel mushy. Seizing up usually means your fork needs to be cleaned and regreased. Take them to a service center to get this done. The elastomer compound may need replacing and this can be done at home.
Suspension forks need to be kept clean. If the area where the slider goes over the stanchion gets dirty it will cause the bearings to wear out. If a bolt on the brace and crown are removed remember to use Loctite when replacing them. Make sure that you tighten them to the correct torque.
Checking your frame
Check to see if the forks are bent. Look from above, the top 6 inches of conventional forks should run parallel with the head tube. A bike with a bent fork will feel odd to ride.
Find where the bottom tube, top tube and head tube meet. Look for cracking in the paint on the top and down tubes. Run your fingers underneath the tubes to feel for ripples. If there are ripples the only solution is to replace the frame.
Check the rear dropout to see if it is bent or cracked. Most steel frame dropouts can be straightened. An aluminum rear frame should have a replaceable dropout. If it is cracked where it joins the frame you may have a bike with a defect. Have the bike shop check it out.