First off, dhaval0603, welcome to Tropical Fish Keeping forum.
Now, to your questions. First being the filter. Is this a Fluval "U" series, by any chance? If it is, you can adjust the flow, and you can also direct it against the end wall to further lessen it. On a 15g tank with plants, I would not recommend a HOB. You want more control over the flow, as this is not only going to benefit the plants but suit the type of fish one keeps in a 15g tank. If you want to change filters, a sponge would be a better choice, connected to an air pump. But what you now have may be perfectly fine with some adjustment.
On the substrate, as rexpepper said, sand is good. Liquid fertilizer is the first thing you need to use, and it may be sufficient depending upon the plants, fish, and water parameters of the tap water (GH or general hardness, which is the prime source of the "hard" minerals like calcium and magnesium). Substrate fertilizers benefit large substrate-rooted plants, but you are not going to have any of these in a 15g. Do you already have the sand gravel you mentioned? Or are you still looking at options? I ask because no mention is made of intended fish, and some substrate fish are better with sand, so this should be decided first.
And on the cycling, with live plants you do not need to fuss over a "cycle" because the plants take up the ammonia and the fish will be fine, provided there are sufficient plants and few first fish.
Hope this helps you get started.