Which one are you more so trying to achieve? Because they are two completely different journeys, I'd focus on one first.
One thing I can say about the both of them, is that you should listen to a lot of the sound that you are trying to achieve, and mimic it!
When it comes to singing, I hear that is one of the best ways, and it has helped me tremendously as well.. Find a singer, or multiple singers that you like, and try to mimic their singing style as best you can. Eventually you'll learn different techniques, just though manipulating your vocal chords to sound like that singer.
Although when it comes to singing, it kind of is something you have or you don't, you can force something not meant to be.. Although you can try to train it, it will never really be that great. Like trying to polish a turd.
When it comes to MCing / Rapping, I believe it's a learnable skill, but I wouldn't say easy by any means. You don't really have to worry about your pitch, or things of that nature when Rapping, it's more of a "Swag" (I hate that word) or "Voice" .. You just have to gain your own voice, and not be afraid to let your voice go where it wants, don't force anything though.. Don't try to make your Rap voice sound like something it's not.. You have to find your voice.
There is so much to this stuff.. But if you want to be the best, I believe pushing yourself to get there is the only option.. Videos on "How to sing" or "How to rap" are usually Bullshit. You really learn the most by doing (Like in production)
Oh yes, one last tip.
Record yourself! This is actually probably the most important tip. When you first hear your voice on recording (Well actually it could take years) before you get used to it.. It will sound completely different than your head voice (Voice you hear when you speak) After a while you will kind of get used to it, and you can use recording as a tool to see how you sound.. Be very critical! If your singing / mcing sounds shit, practice and record again, until you appreciate your vocals, and think they sound somewhat comparable to what you hear in the "mainstream" or whatever.
Good luck
Sorry I can't provide any videos.. Maybe google Vocal exercises.. These are little exercises you can do before you sing or mc which will make your voice sound better and work better.
