I'm new to bikes. I got a used bike that had all the tires and tubes broken so I had to change them all. I changed the tires and the tubes completely two days ago, then I went cycling the next days. Today I went to ride the bike to find out that the front tire is flat (not completely flat, since the tube has a lot of air). Is it just a bad luck? Or does that mean I have bad quality tires or tubes? Or is it just a matter of refilling the tube with air again? Or that I didn't close the valve of the tube correctly? I'm really surprised for this!
How much air did you lose? What types of tires/tubes/rim strips? What tire pressure did you start your ride with?
A few possibilities:
1) Did you replace the rim strip/tape in each rim when you replaced the tubes and tires?
2) You may have made a small hole in the tube when installing either by improperly using a tool to reseat the tire or by catching the tube between the rim and tire bead.
3) You may have a slow leak caused by something you rode over; a thorn, glass, etc. Or you could have a pinch-flat caused by riding over a curb or rock.
4) If the tubes have removable valve cores make sure they're not loose.
First, I'd check that it wasn't a one-off event (like you thought you pumped the tire up but didn't enough) by pumping it up to the right pressure and making sure the valve is closed.
Then, I'd check the tubes and tires for punctures - maybe the rim tape slipped a bit or you weren't running high enough pressure or something (like you got a bad tube - it happens). Over time, you do lose air, but 2 days is too short to be noticable in most cases.
Everything you need to know about flats is here - Take the tube out, pump it up a bit (roughly to the size it would be in the tire, but you can safely go to twice that size) and see if you can feel for leaks. A tub of water, visual inspection or the area around your lips may be useful for this.