bought a 6'6 medium baitcasting rod for fishing a jerkbait. It says it is medium but it feels like it is stiffer than that. What type of line should I get? What lb test? Thanks for any help.

I should have mentioned that this rod isn't just for jerkbaits. I am also planning on using it for a wide variety of lures mainly jerkbaits and topwaters.

I like either fluorocarbon or monofilament/co-polymer for my ripbaits/jerkbaits. 
For fluorocarbon I use either 10 or 12 lb Sunline FC sniper, and for mono I like maxima ultragreen in either 8lb or 10lb.

I should have mentioned that this rod isn't just for jerkbaits. I am also planning on using it for a wide variety of lures mainly jerkbaits and topwaters.

Looks like this post was made as I was responding, sorry. 

In that case I would lean towards 10 lb monofilament, as said above, my favorite is Maxima Ultragreen.  It does have some stretch to it, but it is darned strong. 

I think I will probably want to go stronger than 10 lb because the lakes I fish have fairly thick weedbeds. I often have to pull the fish out of weeds.

I throw jerkbaits and topwaters on different rods as I prefer fluoro for jerkbaits and mono for topwater. If you want to fish both on the same setup then I agree with DTack in that you should use mono. But to be honest I wouldn't throw a jerkbait on anything over 12 lb fluoro/10 lb mono. I actually throw mine on 8 lb Sunline Sniper FC and my topwater on 20 lb Sunline Super Natural Monofilament.

 

Edit: Look into the Sunline Super Natural Mono since you want something stronger. The line diameter of the 20 lb Sunline Super Natural Mono is equivalent to almost all other 14 lb monos. The line diameter is what is going to matter on the jerkbait.

I fish the St. Croix Avid AVC66MF with a soft tip/ Scorpion 1000.

Mine is spooled with #12 Sunline Super Natural.

Suffix 832 40 pound test for me. Add a fluro leader when needed.

12lb sunline supernatural, only mono I'll ever use again.

I like 12 or ten lb flouro...however i have been known to use 20lb braid with a flouro leader from time to time

If you want to use it for topwater as well as jerkbait, I would recommend against full fluorocarbon; I just don't like how much full fluoro it pulls down the topwater baits for most of the time. I would use 20-30 pound braid with a 8-10 pound fluoro for jerkbait and 12-14 pound mono for topwater.

10-12# yozuri hybrid.

12lb sunline super natural

I like 12lb Seaguar Invizx

I have been using D-A-M TECTAN premium plus in a 18.9 lbs and have been very pleased with it. Small diameter low stretch made in Germany.    

I like 12lb Seaguar Invizx

 

Same here.  Good stuff.

8-10lb Yo Zuri.

I'm a braid guy. 20-40 lb on my topwater/jerkbait setup, and you can always add a fluro leader if the water is clear or you want to make the bait suspend a little deeper

X3 on the Yo Zuri. 12 lb should work but don't forget to spray KVD line and lure.  Their Hybrid 12 lb breaks at 19 lbs.

11 lb Sunline Defier is what I use. Amazing line

11lb defier here as well.. If i go to flouro, 8 or 10lb sniper.

Topwater and jerkbaits are on different setups for me but the larger jerkbaits are fished on 12lb Silver Thread AN40, Topwaters are fished on 15lb Big Game.

3 lines I use for hard jerks are 7lb sunline biwako FC, 10lb power pro and a FC leader, 4/6lb Yo-Zuri hybrid. Might seem light to some guys, but I get more casting distance, in clear water I think it makes a difference when you can back way off the school and still bomb a pointer 65/78 to them.          

 

Thin braid and leader requires the least effort, less fatigue overall... keep in mind if youre gonna be jerkin' it all day. Long casts + fast cadence + stretchy line will wear you out.    

Ripbaits - 10-12lb Fluorocarbon

Topwater - Braid

 

BUT if you must use the same combo, mono is really the best and only solution IMO. 10 and 12lb mono can pull some pretty big bass out of pretty heavy weeds. I have pulled 6-8lb bass out of some heavy weeds with 6lb fluorocarbon, not to say it's ideal, but 10-12lb is STRONG stuff, weeds or not.