I have 90k karma on Reddit so here's my experience.
1) If you want a good rep then post memes. The thing is, to post memes you'll need a lot of karma to post them in big subreddits. Once you get the karma just repost funny memes from months ago. Sure you'll get some hate but it's very easy karma.
2) If you just care about karma go to porn subreddits and post NSFW content of others. You'll get more upvotes if you pose as someone else. This will not only give you a lot of karma but followers too.
3) Find some cute puppy pics with Instagram tags, make sure they belong to a real person who isn't famous or something. Post it on some feel-good subreddit like r/aww, make up some fake story about how you saved him from an accident or adopted it. That's the easiest karma you'll get.
4) Once you get some decent karma (30k+) delete your old NSFW posts as they may backfire at you.