Bike tires lose some air over time, but you should not expect to have to put air in the tires every 6 miles. It is not your weight. Most likely, you have a flat tire, though one that holds air for a short time. You need to find the source of the flat or your slow leak will get worse.
Throwing a chain could be rider error, or it could suggest that the derailleurs need an adjustment. You need to take it to a bike shop, or adjust the derailleurs yourself.
Excessive brake squealing suggests the brakes are out of adjustment, or the wheels are not true.
Excessively dirty chain means you need to clean your chain, then re lube it.