For your level of work, neither Photoshop or Lightroom is needed. Just get an account with an online photo gallery/selling site and they should automatically be able to generate lower quality previews and watermark them for you. You don't need software like Lightroom or Photoshop to do that kind of very basic work.

If you really want to do it yourself. GIMP will also easily let you do it and is free.

Also, do keep in mind what others have mentioned. Image theft isn't really going to detract from your sales as long as people understand that you reserve rights on your images. Those who want to use them or get nice prints of them will buy them regardless as long as you aren't asking anything outrageous for them and those who are going to take them won't be dissuaded by low quality and watermarks (though potential customers might be).

Also, don't be surprised if simply posting your images online doesn't generate any sales. Sales is hard. It generally involves grunt work on the ground promoting your images. The site is just an easy place to tell people to go to buy a copy if they want one. You most likely won't get many "walk ins" if any.

Setting up a random website no the web is kind of like opening a store in the middle of a deserted ally. Without driving people to it, nobody will ever even know it is there. Going with one of the bigger sites that does image sales does help some, since people can like your images and cause them to get more attention, but it's still pretty much a luck game more than skill unless you are actively promoting your work.

If it is something you really want to try to get in to, it might also be worth pursuing some better camera equipment and better software for touching up the photos to make a more professional level product. There's a lot you can't do with a cheap point and shoot and basic instagram filters that can be improved on a lot with better software. If you really want to get involved in photography, try picking up and learning a basic DSLR, shoot RAW and try learning something like GIMP or Darktable to learn about photo adjustments and digital darkrooming. It will probably help improve your end result.