You should visit your doctor for a diagnosis. The condition you describe can have a number of possible causes, and in order to treat the condition, the cause needs to be taken into account. Nail disease is of course one possibility, but there are several diseases and medical conditions that can affect the nails. For example, an iron deficiency or a thyroid problem could cause brittleness of nails, while bronchitis, liver disease, and diabetes can cause yellowing. Some sort of infection, probably fungal, is most likely - the symptoms of a fungal infection include yellowing and thickening of the nail. Brittleness is not usually associated with fungal infections of the nail, but the infected nail does tend to crumble. Applying vinegar to the nail or soaking your hands (or feet) in a basin of vinegar mixed with water is recommended for such infections and is sometimes effective, but results are slow to come, and in your case, it is not entirely clear what the problem is.
Only your doctor will be able to determine the exact cause for certain. Even if the problem is only a nail disease and not some underlying condition, spending time experimenting with treatments might allow the infection to worsen and even spread.
answered by G M