Once knocked out, keeping it down will be the least of your problems if you taming in the wild. I suggest to build a primitive trap with stone foundations and pillars. Get an argy and pick one up with right click. Drop it off in the trap and keep a distance - it will fight back, and it will hurt. Once down in the trap, it will very unlikely to wake up before it will be tamed. Good luck.
For a low lvl player a lvl 1-45 is good enough to get you going but when you get the kibble a lvl 116-120/150 should be your main goal. High damage means more metal per hit & weight is your 2nd most useful stat to carry more. P.s taming a lvl 145 ankly now ;)
They are very slow and can't turn around very easily. So come up behind one and shoot it with tranq arrows. You should be able to knock it out before it turns around and attacks you. But don't get too close!! Watch out for other Ankylosaurus too. They are travel in herds