how to level up fast?

Consecutive matches as TeamChevy86 mentions.
Play in a group of friends. I don't think the experience bonus increases as you include more friends like HoTS does, but the baseline bonus% still helps.
Win win win. For the time that Uprising was around, playing the new mode in a group was netting tons of XP. You can do the same with a few Arcade modes or whatever. Additionally - longer games = more XP.
First Win of the Day bonus. Remember to do one AI game per day for the small bump of XP for the day. Around 7-8PM (EST) the bonus should reset, from what I've noticed