It really depends on what sounds you want. If you want sounds that you hear in real life, it will be very hard to recreate such effects.
If it should just be some random clicks noises, etc. then you can start with many different tools, but keep in mind that sound engineering is a profession on it's own, so don't expect it to be the easiest thing ever.
Very useful and easy to use tool (is also mentioned in the Ludum Dare guide) is
sfxr. With just one click you can generate random sound effects and if you find one that you like, you can fine tune it further and then export it.
Otherwise there hard hunderds of resource websites, that provide sounds. One of my favorite one (not only for sounds) is
OpenGameArt.