It's difficult, and there is no one right answer that works for everybody.
I've heard it said that producing and selling off-the-shelf software is a better tactic than physical products, website design or other bespoke services, because that way you have a one-off investment to create the software and then you can sell it again and again for almost no extra cost to yourself ... whereas any physical product, design or other service will require investment or production costs for each and every sale, which obviously means you have to recoup those costs in each and every sale rather than amortising them over the aggregated sales of a single software product.
On the other hand, it may be that you can only sell software to a limited market at a fairly low price – people often aren't willing to pay much for software, which they see as having next-to-zero production costs – whereas for a physical product or bespoke service, you may be able to charge much more, with higher margins, and more profit across the whole production run.
Re-selling other people's products is a tricky one. How are you going to be able to undercut other re-sellers or the original producer, bearing in mind you need to take a cut? Sometimes there may be a way to make money on it, especially if you can buy in bulk, but it's likely to be a more competitive market place because there will be other people doing exactly the same. At least if it's your own product, you can build something unique and distinct from the competition.
A lot of it comes down to your own skill set and capabilities ... what are you able to do? Different people have different strengths, and you're likely to get much further by playing to the things you can do well rather than moving into a new and unknown area with little experience and a steep learning curve.