I really enjoy weight lifting.  I am just getting into it though.  Do you know much about it?
I sure do, I do strength training to develop the strengthen and increase the size of abdominal muscles
Do you know when weight lifting first became popular?
I'm not too sure since weight training is a pretty specialized excersize that targets certain muscle groups. I assume its been around for as long as people have worked out. 
Are there any downsides you know about for weight lifting?   I do not want to injure myself.
Its all about pacing yourself and not lifting too much at one time, You have to start with the small weights and work up, never start with heavier weights. 
Are there any weight lifting competitions that you know of?  
Yeah powerlifting is popular, its basically a strength sport where people have  three attempts at maximal weight on three lifts: squat, bench press, and heavy deadlifts