Rengar jungle
 

I am pretty new to the game and bought Rengar when he went on sale. He is the first champion that I have tried to jungle on. My problem is that I have died to Lizard twice already when my team mates "leash" just one hit and run away. I am not lv 20 yet so I haven't bought runes or anything so that contributes. I see people play jungle and kill it by themselves and I dont understand why I am not able to... Do runes make that big of a difference?

Yes runes make all the difference when jungling the armor really matters till you get them you have to start cloth 5 pot on most jungles

With armor seals, attack damage and armor pen marks and quints with mr glyphs (optional) if team members only hit lizard once and walk away (I use boots and 3 pots) it can take me down to close to 150 health (I also start on wraiths, which take me to around 390 health first), but I use health pots to get back up to around 400. Without runes, it might be impossible to start red without a hard leash. I play rengar jungle a lot, and can tell you that while he may be the best ganker in the game, his jungle sustain is extremely weak even with the 15% heal. Until you at least get armor seals, it might be best to use junglers like udyr, trundle, ww, or cho to start off with until you get those runes. Or you can start blue and wraiths which will get you to lvl 2, and go for lvl 2 ganks, which if successful will allow you to kill wraiths and take on red as lvl 3 with preferably several health pots left over. Best of luck to you, fellow rengar jungler!