Rough seat post height adjustment can be done by sitting in the saddle and pedaling backward with your heels on the pedals. If you don't get full extension of the legs, the saddle is too low. It looks like you frame is too small for you. I'd expect you to be on a frame closer to 20". That being said, you can get a longer seat post to at least get the saddle height right. I wouldn't put much into a bike that doesn't fit. Start putting aside a bit here and there when you can and be on the lookout for a deal on a road bike that fits.