I recommend going to Price Scope and looking on the forums, like Rocky Talky, as well as in the tutorials, AND the Show me the Bling threads. Good Old Gold also has excellent tutorials and video. The posters on PS can also help find diamonds in your range with excellent specs.
The main thing is that “cut is king”. Cut will make lower colours face up whiter and brighter, sparkle more, and so on. It will even make a stone face up bigger, not only because there is less light leakage, but because it won’t be cut just for weight (many poorer cuts are cut to meet a magical mark, like 1 ct, even if it means the stone is too deep, too shallow, etc). I have seen poorly cut 1ct stones face up more like a .5 or .75 ideal/excellent cut. I NEVER EVER EVER see PS’ers recommend going down in cut whereas many are plenty willing to go down on colours, clarity or carat!
How colour sensitive you are will affect what colour grade is important to you. Many people are not at all colour-sensitive and see a H, for example, as white enough for them, etc. Some really want a very, very icy white look and want colourless in the D-F range, and maybe even with fluorescence to make it uber-white. Most are quite happy in near colourless (G-I), some are quite alright with a J, and others (like me) actually prefer warm stones (K, L, M and below) due to the cuts we like (antique cuts). The cut will also affect what colour you want, as fancy cuts show colour more than RB’s, and step cuts usually call for a colourless to near-colourless stone. Stones also tend to look whiter when set (as long as in the right metal for that stone).
As for inclusions, on Price Scope many will say going down to SI1 is okay as long as it is eyeclean to YOUR standards, to save cost and maybe go up in other more important to you areas (size, colour). Some trusted vendors online can help pick out stones for yo this way, but you should ALWAYS check the stone yourself. Your eyes may be different than someone elses. Some are more inclusion sensitive though and would never go below a VS1 or VS2, others need VVS1 for mind-clean aspects. It also depends on the inclusion – they type, the location of it (is it able to be covered by a prong), does it affect performance (ie does itblock light return), does it weaken the diamond, does it look cloudy and lifeless? Personally, I would be okay with SI1 as long as eye clean within about 6″ and would prefer a twinning whisp of some sort over a obvious carbon spot or surface breaking fracture. I would not recommend going below that or S12 for an engagement ring stone. my exception would be if that grading was due to it being an antique cut with chipped girdle issues that could be protected, etc. But yeah, while I love lower colours, I personally am pretty picky when it comes to seeing inclusions, (as well as when it comes to cut).
I would NOT recommend just going to any old jewelery store in the mall to look. Often they are not GIA graded stones, and grading by, say EGL is notoriously bad (an EGL H, SI1 may be a GIA J or K, I2). I would find a reputable dealer, or somewhere where you can look at Hearts on Fire diamonds, etc. If you can even travel somewhere to one of the Pricescope vendors in NY, etc, I would even suggest doing that to get a good look at some great stones, have a great experience, and really learn a heck of a lot!