According to some manufacturers, engines can use up to 1L in 1000 miles. However, that would alarm me and I've never seen it in practice in a healthy engine (ex mechanic). The first thing I would do is top it up on level ground and see how it goes over the next 2000km.
With 25,000km on the clock your car is unlikely to be burning oil - by the way if it was using 1L per 1000 miles as some manufacturers suggest, you would be unlikley to see smoke from the exhaust. Its not impossible to have a major fault but highly unlikely. As you say, you don't even know how to jack the car up so not being rude but it is entirely possible that you did something wrong when you changed the oil.
Did you have the car on level ground?
Did you check the level after filling it and before you started the engine? If so, it would have dropped considerably as the oil ciruclated through the engine and filled the filter.
As I say, get it in to level ground, top the oil up and check it again in 2000km. When you top it up, wait 5 minutes before checking, when the level appears correct, run the engine for a minute, wait 5 minutes and check again.