green-man:
...i dont know the specific type of the led nor the transistor - how can i find it out ?
If you don't know what devices you are using then you need to do some more investigation to find out. The transistors should have some kind of markings on them that you can search on. For the IR LED it may be a case of searching for similar devices until you can find one that looks like what you have.
Where did you get your components from?
You need to find the data sheets for the devices you are using. All the required information is contained in the manufacturers data sheets.
Post photo's here. Someone may even be able to tell you what you have from a visual. There are guys here with years and years of experience and knowledge.
EDIT: For example, I've just looked at a 5mm clear IR LED on Sparkfun that quotes 1.5VDC forward voltage and 50mA current. You stated 12VDC supply and 33R resistor. That is over 300mA if you fully saturate the transistor. Assuming you are using an NPN; your transistor has a Base resistor of 2k, from a 5V Arduino output, meaning 2.5mA into the Base. You'd usually want 2-5 times the saturation current, so IF your transistor is capable of switching 300mA it would need a Hfe of around 120-150 @ 300mA to even have any chance of being saturated. Realistically it would need to be in the 300-700 range, or you'd need a lower Base resistor and a correspondingly more Base current. If using a cheap transistor, it may well only be rated 100mA and Hfe of 30 @ 100mA anyway, so you could have damaged the transistor and/or LED due to overcurrent. Without knowing exactly what you are using then then it's mostly 'what if', assumption and educated guesswork.