All tyres lose air pressure, it’s normal. Now, if you’re tyre is flat , and it wasn’t yesterday, Then you have a puncture…and I will cover that in another blog.
Narrow/high pressure tyres lose pressure the quickest. Those tyres can lose 10 psi overnight. Again…This is normal. It is best to top these up before every ride. Wider/lower pressure tyres hold air better, but you still need top them up every week. I have found that some innertubes are better at holding air than others. Schwalbe tubes claim that you only need to fill them once a month! I have found that they do hold pressure for quite a while, but check them anyway.
My best advise is to get a good pump…and use it! I have nice pumps starting at $25.00 and up. All of these pumps will work with both presta and Schrader valves, and have built in pressure gauges to make it easy to get the correct pressure every time.