Transmission issue

I have a 2004 trailblazer 4.2 v6 I noticed a
few days ago it is shifting hard from first to
second as well as no overdrive and it seems
like the gears are being ran out before it does
finally shift. A code came up for a
pcm(powertrain contol module) but I’m
hearing different things from people on what
to do for it. Kinda lost on where to even begin
and don’t have the money for a new
transmission just yet. What should I start
with?