Whenever i get into my 2003 Tahoe Z71 it

Your going to have to take it to a transmission shop that has a tech 2 scanner and have the transmission scanned for DTC codes. It sound like the #1 servo is not engaging which can be caused by a dirty transmission oil filter and fluid. The fluid retention is to high casing the #2 servo's to stick until the transmission line pressure raises to force the transmission into gear down to 1st gear. This can also be caused by a faulty TPS reading. Try this for now, place the ignition key's to the run position and pull the ECM (BATT) fuse out for 30 second, then place it back in. Start the engine, it may stall, but just let it and don't touch anything. The ECM is relearning the engine and transmission, just start it after 5 seconds and let idle for 1 min. Go for a test drive and everything should be good as the day it came off the assembly line. If there is any problems, there all going to be new in the ECM history and the Tech 2 scanner will pick it up and find the issue if any. Good luck and keep me posted. be glad to help. In the meantime service your transmission if it have not been done, first transmission service was 30,000 miles then every 100,000 after.