If they lose air at different rates, one has a slow leak.
A basic floor pump should be $25. Be sure it works with both presta and schrader valves, the walmart ones sometimes don't do presta.
Glass causes most flats. Go over your tires once a week and dig around in any little cuts you see with something sharp and get the glass out. Get a patch kit and learn to use it, as well, and you will save a lot of $$$ on tubes.
The main sign that a tire is underinflated is that you put the pump on it and it takes more than a stroke or 2 to get it up to the correct pressure. So go get a pump and quit trying to guess about this stuff.