Krdax - you should post a replay of one of your games if you want some pointers. It's generally unwise to focus on a chogath in a team fight as he's often the tank, so I wouldn't be surprised if someone else is focused ahead of you. Good players do not focus the tank. Also, if you aren't the target in team fights, then that means someone else is. You can come in extremely handy here by using your silence at the right time along with rupture and your ult.
Re: runes, I think you should go ahead and at least get some. Try the trick I listed in the new player tips. Even if you only choose to get level 2 runes (using the trick) you'll get like 5 for next to nothing in IP. My take (and what I did) was eventually just to pick up some runes and add them to my summoner sheet as soon as possible. Even if its just a few random buffs, its certainly better than nothing - and you can always trade them in for different runes.
Re: inviting folks to your games... well, the matchmaking is a bit uneven at times. I'm not really sure what the formula is. What I can say is that when I queued up with someone that had no games against humans, I had 46 kills. That's like vs AI silly numbers. So, I ended up with an incredibly easy game, but we lost because my team really didn't know how to win it, even with me killing everything in sight. If anything, though, if you queue with someone else that has plenty of games, its probable that you personally might have a harder time of things as you could end up with someone on the other team having the same amount of experience as whoever you queue with. It's still good to queue up with people you know though, as knowing what to expect from your teammate goes a long way.