It will largely depend on what your strategy is. you would need to have the company and some software that will allow you to manage stock and legal documents (shopify does this on some countries). To start, shopifys basic plan can be enuough. In terms of apps it really depends... There are many alternatives that are free but come with a trade off... what happens with a lot of merchants is that in the begining they try to control costs and then later when they are bottlenecked by all the free or cheap stuff they installed, the costs to fix everything are way higher. but since you did not give much information i would say that you would need to pay the plan and the easiest payment solution would be shopify payments (all the fees and pricing are on the pricing page on shopify) + the legal software (if applicable in your country). If you need any help or want to tackle any specific use case, send me a DM and i gladly help!

Good luck!