Dust is a great place to start off especially for those who have played CS:S. It's familiar ground for you to get a hang of the weapons, and other basic skills you'll need to play the vanilla maps. I feel that people coming from TF2 (such as myself) have an extra challenge because the game play is so radically different. I jokingly said before our scrim with the TF2 division "We won't have to kill them because they'll be busy spy checking." =p
It will take you about an hour (as some have mentioned) to get a hang of the basics, but you'll take off from there.
Don't forget, all of you who are just installing INS you are all welcome to attend our Boot Camp on Thursdays. This will teach you to become a better team player, and for those of you who have even just a few hours of INS know how pivitol a cooperative team can be in determining which side wins. On Sundays we have Sunday school which isn't as intense as Boot Camp. For instance, there is usually one lesson for the entire session, but it is also much more casual. This atmosphere leaves room for questions to be asked and answered where Boot Camp has a tight agenda which must cover a lot of information in a short time; therefore, limiting time for Q&A. Both are very informative and helpful, please check them out.