I started speedrunning only a month ago, and just submitted my first run tonight. If it gets accepted, I'll be in 80th place of a pretty popular game (Contra on the NES). I choose Contra for various reasons:

1. It's a game I love to play, and have played hundreds of times during my childhood, so I already was kind of a "half-speedrunner" at it and didn't even realized it (XD);
2. It's pretty short (10 - 15 mins);
3. There are various guides for it, like the one on YouTube I studied the most, from AngryLanks, the current WR holder;
4. There are various categories to speedrun, so you have some variety on that;
5. It's emulator friendly, as long as you follow the emulation rules and use the acceptable emulators, which means it's way easier to practice those nasty segments with the use of save-states until you nail them. . So yeah, most importantly, choose a game you won't get easily bored playing. For starters, I would recommend Super Mario World, it has some short categories and a very active community, which means it would be easy to find some detailed guides for it. And while watching and studying guides and stuff may sound like a chore, it was actually the most fun part of it all to me, watching an excelent runner completely dominating the hardest parts and doing stuff I didn't even realized was possible. And most importantly, teaching wannabe runners step by step how to do all of it.

E saudações BR aí, fera! ;)