You have a lot of things that you're asking.

First things first (and the biggest mistake I see app developers make is)
1) Have you validated your ideas with your target customer(s)?

Let's see you've realized this is what you customers ACTUALLY need and they're willing to pay for it.

2) Next step would be to create a minimum viable product (MVP). This will show you how and what they use it for.
Example:
When instagram first released their MVP, they advertised it as a geo-referenced based social network. What people really ended up caring about was the filters they could use on pictures. So they pivoted and moved forward with that (amongst other things).

If you don't know how to code, you can either hire someone to create the MVP for you, learn to code yourself, or get a technical co-founder. Pricing varies from $20/hr all the way up to $300 depending on who you're using. For an MVP, I suggest going with a site like toptal.com or Elance.

3) If your MVP proves to be successful, then you need to fund and execute an actual launch. This can be done through VC funding, bootstrapping, loans, money from family and friends, etc. The execution is the MOST critical part of your app. You have to build buzz, create a sense or urgency, and get downloads. I don't know what your ideas or what your app will do specifically, so I can't talk about EXACTLY what to do.

First things first, I would go to your local library and check-out a copy The Lean Startup or The Personal MBA or watch videos on youtube or look here: https://www.startups.co/education

A lot of the videos and books can answer majority of the questions you asked above and give you a lot clarity on the the topics I wrote about above.

Additionally, I'm more than happy to speak with you about this here: https://clarity.fm/taylormitcham

Receiving feedback from your clients/customers. Engaging with them as much as possible. Learning about things that worked and didn't work so you can tweak your product or offering for their actual needs vs. what they or you thought they wanted/needed.

Actually there is a site called the http://www.themembershipguys.com/
They have a lot of advice.

As far as starting a membership site, you can integrate the wordpress Buddypress plug-in with woocommerce or another payment provider. As far as templates, there are a lot out there for membership sites. Without knowing EXACTLY what type of site and more specifics I can't give you a specfic recommendation.

If you'd like, we can schedule a time to chat, and I can get more of a feel for exactly what you're looking for and give you a more targeted recommendation and plan on how to get started. https://clarity.fm/account

Take a SMALL deposit upfront (the actualy - not marked-up cost - of the materials. That way if they skip out on you at least you didn't lose anyting. Second, you won't give them back their car until they pay, so you always have their vehicle as collateral.

The point at which you buy materials will depend on how long the lead time is before you get them, and how quickly you're guaranteeing the service. In this day and age, the faster the better. How long will it take you to paint the vehicle?

You should schedule their appointments before they pay. You can start with a free "consultation" where they bring their car in, you walk them through the process, and explain to them how its going to work.

I would have the option to accept deposits online, but anything outside of that, I would not.

In person, you need to be able to accept cash AND credit...especially credit.

My suggestion is to have a process like this:
1) customer calls asking for a paint. Likes what you say, agrees to use your service
2) Collect the deposit
3) Go get the materials
4) Schedule a time for them to drop-off their vehicle AFTER you've purchased the materials so there's less downtime
5) Paint the vehicle
6) Customer picks up the vehicle and pays

I'm more than happy to speak with you on this further. Feel free to schedule a follow-up call and I can walk you through this step-by-step: https://clarity.fm/taylormitcham