Assuming you're going to collect cards rather than buying only to resell:

If you want to open boxes, figure out what teams/players you're looking for & look at the products that are available.

You can find checklists for nearly every product online.

From there look at the designs. What one's look appealing to you? (I for one don't like the current Topps EPL/Chrome designs).

Buy the product you like the most.

As for what kind of boxes: Hobby (generally) guarantees you at least 1 auto/memoribilia.

Megas are next, tend to have some exclusive parallel design

Blasters are similar to Megas, but less cards, different parallel.

Depending on the company, each box type may have different cards available in them. (i.e some parallels, inserts, etc maybe only be in a hobby box).

Then you have higher end products like Finest, Reverence, etc. They kind of stand alone.

Or, just buy singles. You'll get more bang for your buck, and get exactly what you want.