2002 Chevy Tahoe A single mother of 3 inneed of help please

My transmission slips 1st gear but im able to play with the gas pedal to have the 1st gear kicked in. I dont know how I do it, I just got used to it. Ive been driving around with it like this for about a year now and I bought the vehicle (used) in 2005. I want to know if its to late to flush the trans? Will it make a differents? What would be causing the slippage? What will I need to perform a transmission flush? What type of transmission oil that would be best to used and how many bottles will I need? What type of formula that is safe to pour in the transmission to clean that gunk build up? I really appreciate your time & answers. Thank You