I'd be happy to help you out at no charge. Happy to see fresh blood in my favorite aspect of SL. To answer some of what you've posted here:
Land for the establishment: You don't necessarily need land to start a business. In fact, unless you intend to sell furniture, homes, or other things that would be better-suited to a rezzed, inworld demo, I would advise starting on the marketplace first! You can set up a marketplace storefront for absolutely zero upfront cost, and zero upkeep costs. The only cost to you for marketplace sales is that Linden Labs takes a 10% commission fee out of every sale you make. It seems a bit steep, but unless you're also planning to sell a lot of big-ticket items, this will often end up cheaper overall than an inworld store. Plus, many consumers like the convenience marketplace shopping brings. Even if you do opt for an inworld store, I would HIGHLY recommend setting up a marketplace store as well to get more eyes on your products, and more feet in your store. To begin setting up your marketplace store, log in to marketplace.secondlife.com and click the "My Marketplace" button at the top of the screen, then click "Merchant Home" on the dropdown.
However, if you ARE selling things that need to be rezzed, rather than worn, to be seen, or you're dead-set on having a physical presence, you can always buy or rent land. If you'd like to own land, you'll first need a premium membership through Linden Labs, as well as some starting funds in the form of lindens. Once you actually purchase your land, you'll also need to pay for the appropriate tier level for the size of land you have. All premium accounts have 1024sq m of free tier to put towards a parcel. It would technically be possible to squeeze a small store into a parcel this size and not have to pay any monthly fees except the cost of premium alone, but you're likely going to be making some sacrifices in your build. Further, premium membership gives you L$300/wk, plus a L$1000 stipend if you've never been premium before, and you keep the service for at least 45 consecutive days. You can use this money to invest in your business. You can find parcels for purchase by checking the world map in-game and looking for the $ icon around the map. You can find more information about tier pricing here, and more info about buying land here.
Alternatively, you could do away with the whole process of premium membership, and tier, and all that stuff, and rent land from another SL user. Renting can give you a lot more options for store types at any budget, from as small as a 10 land-impact allotment to place on a pre-built kiosk, all the way up to renting full sims you can build in from the ground up. Just be sure that, whatever or wherever you're renting, you check the land-owner's rental policies to be sure they allow commercial operations on their land, as many do not. You can find land rentals by searching for "rental" or "mall" either on the SL website, or in the search function in-game
Building a store - Whether you're renting or buying, you're going to first need vendors to place your objects in to sell them. CasperTech offers amazing vendors that are well-loved across the grid for the ease-of-use and features they offer. They even have some free models if you're really on a budget. I highly recommend you give them a look. Next, you'll need to actually set up shop. If you're renting a tiny kiosk with only a few LI to work with, I would recommend rezzing just your vendor(s), a sign, and perhaps something people can click to get a link to your marketplace store and/or join your group, if you have one. If you're renting a predone storefront in a mall, there will likely already be walls in place, so you'll have your full allotted land impact to place down vendors, group joiners, signs, and decorations. You can find all of these things on the marketplace website. If you're renting or buying an empty parcel, you'll also need some sort of structure to house your merch. I've seen stores as simple as a big, flat wall with a bunch of vendors, but you likely want something flashier. There are loads of pre-made store buildings on the marketplace, so find one that meets your size requirements, land impact allotment, and budget, and place it down. You could also build a structure yourself, but unless you're good at 3D modeling, this isn't advised, as you'll end up spending a lot of your land impact on prims, and you won't get a very nice looking end-product. Once you have your building down, you just fill with your vendors, group joiners, signs, and decorations as before.
Extras - You'll want to get an idea for your brand's theme, logos, and overall style before you begin making any type of storefront, on the marketplace or otherwise. Making a 2D logo and sign that you can use on both the marketplace and on a flat prim inworld is a great starting point, assuming you haven't already done that. Choose (or create) items and colors to decorate your store that compliment the overall aesthetic of your products. Look to other successful inworld businesses in your market sector for inspiration.
If you have any more specific questions, i'd be happy to answer them. Alternatively, if this all seems like a massive headache, i'd be happy to help you plan and set things up inworld, while teaching, for a bit of compensation for the time spent.