The CTA has a nifty Trip planner at:
http://tripsweb.rtachicago.com/
Beginning at 11:00 a.m. the trains are:
Depart ORD 11:07 arrive Lake/Clark 11:55
Depart ORD 11:15 arrive Lake/Clark 12:03
Depart ORD 11:22 arrive Lake/Clark 12:10
Depart ORD 11:30 arrive Lake/Clark 12:18
Depart ORD 11:37 arrive Lake/Clark 12:25
and so on.
The cash fare is $2.00
No note that the Blue line is infamous for delays, but this is the published schedule.
The CTA station is about the same distance from Terminals 1/2/3 since its sort of in the middle of the triangle they form. If you arrive at 10:45 and have no checked luggage you might just make the 11:07 unless you're at the tail end of concourse C. But the trains leave every 7/8 minutes at that time of day.
Just follow the signs to the station.