I would agree with fotogk. The best way is to shoot. Whenever I invest in a new piece of glass, I'll poor thru every copy in the store until I find one that I'm 100 percent happy with. Sigma's newer lenses have produced some awesome results, and for those with crops, the 30mm prime is said to be one of the best primes to fit. I tried a few, and for photography the lens was awesome. But for video, it had a few issues, so I went with the Canon 28 f1.8 instead ( which is awesome ). I couldn't afford a nice "L" series, or piece of Zeiss glass until I had brought in a few clients but I still had to find a way to get started.
Today, I am madly in love with my 70-20 f2.8. I use it more than my Canon (don't tell!)
Good luck!

I had a Sigma 28mm-80mm zoom back in the 70's that now resides at the bottom of Yellowstone lake after filling with moisture on a foggy morning. Poor sealing has been a problem with them for many years, but their higher end lenses are quite good. I also have a Sigma 400mm APO F4 that I have carried for over 20 years and it is an EXCELLENT long lense for wildlife work, and is well sealed against moisture from my experience. I also have a couple Tokinas, buy their "Gold" trimmed lenses, not the cheaper "Red" trimmed ones and you will do good with them too. I have no Tamrom experience but have seen good as well as poor quality from them in the lenses used by others around me. Even the lenses from Nikon and Canon came in varying degrees of quality.
An old adage: "Your don't need to buy the best there is, but buy the best you can afford." Follow this simple rule and you will have a great time with your photography as well.