I have a 2nd broken spoke after 300km on a new bike, is it the wheel or me? I am new to riding. I rode as a kid but stopped for many years. My Physio recommended riding after a leg injury while running, so I went down to the local bike shop and brought a new bike. I am now 100kg (220lb) so I need the fitness. Bike is a lower-mid level hybrid bike costing AUD$550 (Malvern Star Sprint 1).

When taking it out of the garage for a ride after doing about 150km (90mi) on it I noticed the rear wheel hitting the brakes as I wheeled it and then the broken spoke. So I took it back to the store. They told me that it was 100% my fault and it wouldn't be covered by manufacturer warranty, and I must have hit a pothole or something. They told me that they will still fix it for free as a goodwill gesture and some tips about not riding off the tops of gutters (I didn't), avoiding potholes (I try), and taking off in too high a gear (strange to me but ok). The only thing I can think of that I could have done was a flat gutter that had a bit more bump than I was expecting but nothing I believe that could have caused that.

A few rides later, about 300km (190mi) on the clock, it happened again, broken spoke and a bent rim as I was wheeling it out (both times I didn't notice wheeling in, but I didn't really check). I have been trying to follow the advice but some potholes I didn't see, but again nothing that I think was hard enough to do damage.

I know if I go down to the store again, they are going to charge me for the repair as they say it was my fault again. If they do, it probably means the end of riding for me as I can't see how I can avoid damaging it. So is it my fault, am I riding wrong or something wrong with the wheel? What should I tell the store?

Edit: Thank you all for your great suggestions, this is a very welcoming community. I took it to another independent bike store that I found with good reviews to do a wheel rebuild. Talking to them they said bikes at that price point had good frames but skimp on the wheels to get the cost savings. He recommended that I get a completely new wheel as the rim and hub weren't that good either. I don't know if he was trying to upsell me but he seemed to know what he was talking about. Total cost AU$200 with installation.
Not enough points to comment, so here is an answer instead, based mostly on my own experience.

I've been using a bike as my main, if not sole, mean of transportation for the last 20 years or so. During that time, my weight changed a lot (from 80 to 130kgs (176 - 286 lbs)).
When I was 80, I nearly never broke a spoke (maybe once per 10 000km), and only after a big impact.  

I began to break spoke significantly more often when I reached 95kgs. But it was very dependent on the bike. On some it never happened, on some other it happend more frequently (once per 3-4000km). On one (very cheap) bike, I even broke 5 spoke in 10 km without any kind of impact on the wheel.  

At 130kg today, with a 400€ bike (Trekk 7.1), I broke spoke even more frequently, like once per 1000-1500km. Always on the rear wheel. I changed all the spokes on my rear wheel to more thick ones (I can't recall previous and current diameter though), and since then and 8000km, I didn't break a spoke once.

I spoke about it to 2 repairmen in a bike shop, both said it was just that my weight was too heavy for the bike.

So, to answer tour question, I think it's both you and the wheel. You just have a wheel with too weak spokes for your weight. It can be because of the base quality of the spokes, or because of a bad set up of the spokes tension though.