So if I understand you correctly, you are sitting on your bench, getting under the bar, and then standing up with it? If so, do you then walk forward or backwards so the bench isn't under your legs for the actual squatting? Are you re-racking the weight by then walking back over the bench and sitting back down?
All of the above sounds pretty dangerous. For one thing, you are effectively doing a bizarre box squat, which is a somewhat advanced exercise that requires very good squat technique. As a beginner, you're likely to hurt yourself trying. Moreover, you run a good risk of losing your balance, tripping on the bench, etc, which is never good with a heavy weight on your back. Also, you have to understand that whenever you lift, you run the risk of using too much weight or going for too many reps and being unable to finish one. It's a normal part of training and if you had a proper power rack with safety rails, it's not an issue: you just leave the bar on the rails. Without one, you'll have to dump the weight, which is a bad idea if you don't have bumper plates and an even worse one if there is a bench near you (or worse yet, under you).
Finally, depending on how low your bench is, this is going to make it tough to lift heavy weights. Due to the stretch reflex in your muscles, you are quite a bit stronger going from standing -> full squat -> standing than you are going from full squat -> standing. The part of the squat where you descend into the hole stretches your muscles out, storing energy in them that can be re-used when you rebound back out of the hole. If you start from a dead stop at the bottom of the squat, you don't have any of that. Not a big deal with light weights, but it'll make life tougher as things get heavy, especially if you ever wanted to max out (which I wouldn't recommend without a power rack anyway).
I strongly recommend getting a power rack. It's a very versatile tool, as you can use it for squats, bench press, OH press, pull-ups, inverted rows, rack pulls and more. Moreover, it makes all of these exercises safe to do on your own. You can often find them on craigslist for dirt cheap.