Make them come to you, advertise you have the download portal for sure but don’t put software on the site unless you have permission. What I would say though is you could start things off by reaching out to open source projects, they are more liberal in allowing their stuff to be distributed so you could built up a pretty good index of quality software before you approach software creators to add their own stuff, it’s quite a challenge indeed.
Getting software developers to start submitting to your site is probably the first challenge, as user submitted content a much easier way to grow your site in the long run. You might want to think about contacting some smaller developers and make arrangements to get them added to your site, or even sponsor their software on your homepage. A couple of sites that are smaller and more something you would start with would be something like these: http://best-php-scripts.com/
The first one has done a pretty good job with finding sponsors to pay a monthly fee, and seems to be a pretty popular site. I get a good number of referrals from them. The second is more windows type apps. They offer links to the downloads, and seem to be generating some revenue through google adsense. I believe they may also be letting developers sponsor categories.
So hopefully that gives you a couple of ideas. Sometimes it is better to model yourself after something that is more easier to duplicate in the short term, and then hopefully in a few years you will be the next download.com! Good luck.
You put the file yourself, using a PAD file. Shareware makers create this file so you can automatically upload the software and include a description. Google for “PAD files” and you’ll learn all you need.
Now, a download site is all about SEO. You can’t compete against download.com, but you can rank well in Google if you chose a very specific niche or a country without strong competitors.