The girl in the fish store sold me some plant " A........." that looks and
probably is a weed and said no one can kill it. I killed it.

Never listen to 'some girl' in a LFS.

I killed everything too before I started to read newgroups and websites.
I almost gave up. I then found a selection of plants that would be good
for my tank setup, and crammed the tank full. An odd new plant is still
an experiement since I'm a newbie, some things just don't seem to take
when they should. However, I now certainly can grow stuff.

My first mistakes whe

1) Buying crappy LFS plants
2) Not packing the tank with plants when starting out to out compete
algae
3) Not giving plants a chance to establish (leaf drop when they are
transplated is not too uncommon and they can grow back, like crypts.