MP3 to MIDI converters will give you a very bad time. They're very unreliable for numerous reasons and the result will very very often end up in a range from absolutely terrible to "what horrible sins have I committed by bringing you into this world?". You'll be spending a whole lot of time fixing the monstrosity of a MIDI you'll create with one, way more than if you had just tried to create whatever song you wanted by yourself even with no experience.
For the suggestion of trusted MIDI websites... well, If we were to link to MIDI websites, we'd also have to consider questions like "what MIDI websites can be trusted?", "How do we ensure that they don't turn bad later and users give us the blame?", "What kind of MIDI websites?", and of course we'd have to make sure it's clear that we're not affiliated with the websites we link. That'd be a bunch of extra work when, frankly, it's very easy to find MIDIs. A quick google search of "-insert song name here- midi" will usually do the trick. Also...
Incoming rant-ish opinion
While MIDIs are a quick way of creating the MMLs you want, fluffing them into beginners faces or making it look like we believe MIDIs are the main way of making MMLs might give users the wrong idea as well as risk making beginners rely way too much on them. Not everyone uses MIDI; a lot of people transcribe/remake sheets or do it by ear, both ways that arguably can be seen as "better" since it's harder to take short-cuts and therefore can give the composer more musical experience, usually allows for more freedom and that composers usually have to put more work into their MML and thus grow to care more for the quality of it. Beginners who use MIDIs also tend to do it in a pretty straight-forward way without many personal "touches" or experimentation, which makes the likelihood of identical or near identical submissions (whose MML originates from the same MIDI) rather high. In a bunch of cases, MIDIs might also be a bad idea for beginners if it's a complex MIDI that'd give the beginner problems. This doesn't mean in any way that it's bad to use MIDIs or that it makes you less of a composer, God no, I use some MIDIs myself. It just might be a bad idea to make it seem like that's the endorsed main way of doing things, when it might not be right for everyone.
That's just my take on it though