Learn a COMPLETE count system
I would recommend KO or Hi-Lo for beginners and recreational players. There are some slightly more powerful, but more difficult systems which are worthwhile only to the more serious, higher stakes players. It is much better to play a weaker system well than a stronger system poorly. One little mistake easily offsets the tiny gain that the stronger system offers.
I stressed "complete" in my post title because learning the actual count is the easy part of counting and only the tip of the iceberg. It is very critical that you learn to spot good games (good pen, favorable rules, no table crowding, etc), and learn about standard deviation, risk of ruin, EV, bet/spread sizing, and bankroll requirements, and how all of these are related. It is also very important to have self discipline, and be able to play like a machine according to the system.
Beginners commonly read one book & learn a count, then hit the casinos with a $100 BR and expectations of winning a fortune. They do not understand ROR and that the game is a long grind, with normal, huge, std dev swings both up and down, and get wiped out. Others simply don't have discipline, and after winning a few sessions in a row they start feeling bullet-proof and over bet instead of following the original plan. Or they lose a few sessions in a row and either chase losses or get scared and quit doubling when they should.
And of course as one moves up to higher stakes, he must learn to recognize heat and to use some cover techniques, and many other finer points of the game.