Why does my tires keep going flat on my bicycle?

I love my bike and I ride it all the time. Everyday lately it seems I have to change the inter tube. I just bought 4 new inter tubes from wal-mart. Cheap ones were about 5 bucks a piece. Every time I change them. The next day I seem to have to change them again. And here its really dirty in the streets. Nasty alcoholics and gangsters throw their bottles in the road and let the glass shatter. So im constantly riding over this stuff by accident. "im not answering my own question here" Because that can't be the problem every single time. I ride a mountain bike and lately I just don't know. Its only about 5 months old. Its still fairly new. But ive changed the inter tubes like 7 times this past week.