I haven't sped run before but I have been binge watching speed runners and I want to try to dedicate myself to a game and speed run it. I was thinking Mario 64 or Breath of the wild because I have been playing those two lately. 
Does anyone here speed run or have been to speedrun conventions like AGDQ? 
I might suggest starting with something less lengthy, like as was mentioned before, the original super mario bros. What you're going to run into with speedrunning a game like BotW/Mario 64 is that you have to input a LOT of time for a single playthrough (assuming you're doing a full run, not these 5 minute deals), while having very little experience. Progress will probably be a bit slow, and you may not see improvements in great strides due to the length a single run requires.
Instead, start with something much more simple. If you begin by speedrunning the first level of super mario bros. (as was mentioned), or even just completing all of world 1, there are several benefits.
1. A single playthrough requires very little time. Therefore your rate of improvement has the potential to be much greater than having to spend 2 hours just to finish a single speedrun (if you can finish and learn from 10 speedruns in the time it takes to finish 1.5 mario 64 speedruns, you're potentially seeing many more small improvements in the same amount of time). This will be much easier on the morale, and for someone just starting out, it's important to have more steady improvement, I imagine.
2. If you fuck up a speedrun completely in a short game, no big deal, a very small investment. Fuck up your Mario 64 speedrun 45 minutes in and you wasted a lot of time. That's going to feel like a big setback, I imagine. Again, morale.
3. I think starting small is going to teach you more about speedruns faster than a marathon. After all, speedruns (in games like Mighty Switch Force at least) are all about shaving those little seconds; games with shorter runs demand that every millisecond be saved. You'll have to pay attention to details, and get really tight on the controls; one mess up in any game is going to potentially cost you a lot of time. These transfer well to longer runs I imagine. as they demand the same strategies over a longer period.
3.5 - Also consider that with something short, there are only a limited number of ways it can be ran at the fastest time. This means it's very easy to watch a speedrun (or your own) and take notes, or even read another person's analysis. What techniques do they use to shave off time? Are they conventional or unconventional? What kind of skills do they require to perform well? Are there any methods of practice they use to perform consistently? Consistency for longer speedruns is key, so you also have to focus on perfecting the basics as well.
TL;DR - I feel as if tackling a game like BotW/M64 is like trying to run a 50 mile race when you've never ran a marathon before. Instead, start with a 5K. Work your way up. Of course, only you know what games you like and don't like, and to what degree you can handle these learning curves, how fast you learn, etc. I would just think it would be better to start with something small, short, and sweet, then build upon it.
Of course, again, I guess these games can be completed in a short amount of time depending upon what is achieved in the speedrun, if there are glitches, etc. But I'd highly advise starting small if you can help it. I like Mighty Switch Force; levels don't take any more than like 3.5 minutes, and controls are limited and tight, so you can very quickly learn how to shave time off your runs as you practice over and over, and learn to hone technique (learning how many shots different enemies take for instance, and using the exact amount as to not waste any extra time, for example).
