What will always get you is doing something unusual. When working, always have a safe procedure for moving and be very careful when doing things a different way. When at home or at leisure, don't go beyond the limits of what you're used to doing without planning and prep. And know those limits! I've pulled my back a few times before and I wasn't doing anything more than usual, but I was doing it differently, and it got me.
I have a weak back already.. is it advisable to start lifting heavy things still?
Last time i did i felt like my back got worse. Poor technique perhaps?
People default to thinking about big heavy deadlifts, but it really doesn't need to be. The muscle activation, posture, etc from light deadlifts is really good. It strengthens your back. (Talk to your doctor blah blah blah.)
Start with a bar, get your form down. Add weights very slowly (like you should be getting frustrated by how little weight you're lifting). Keep adding making sure form stays perfect. If back starts hurting drop down in weight, and start doing back and core and glute strengthening exercises. Continue.