why is 2000 automatic monte carlo slipping when shifting into 2nd gear

Check your transmission fluid level.

You have to check it running and warmed up. If it's brown and smells burnt get it flushed(I highly recommend BG). If its red like kool-aid and just low fill it up. However, the fluid is probably leaking out somewhere. Get it fixed or it will ruin the transmission down the road.

Js08016 could be right but if the 1-2 shift solenoid was failing it will usually kick on the service engine soon light. Is there a S.E.L. on?

I had the fluid checked today and he said "not a great color - may need to replace transmission and he would do it for $900."!! Someone else thought it might be the throttle position sensor or transmission solenoid. Service engine light has not come on - yet. How do I check to know if it is the throttle position sensor or trans solenoid? I have a hard time getting people to be honest about it - I'm a girl and don't know jack and they know it!! Thanks for all the help...

Well that's not good. You can always get a second opinion from another REPUTABLE mechanic. The throttle position sensor won't make the transmission slip or slam into gear.

Just so you know $900 would be cheap to have a transmission rebuilt. Make sure you get that in writing if you end up going that route. Make sure there not just swapping your old transmission with a used one from a junkyard.

The $900 is his labor, I am to supply transmission. What about transmission solenoid - is there a way to check that? I may have to go dealership route even though it may be most expensive

Do not go to a dealership. All there gonna do is try selling you a new car or a brand new transmission. Go to a independent mechanic or a trusted transmission specialist. To replace the transmission solenoid you have to remove the trans from the car and open it up at that point you have it rebuilt and get a warranty with it.

Js08016 is correct. A 2000 monte is either going to have a 3.4 liter (3400) or a 3.8 liter (3800).