Seconded on KBoards being a great resource. I can also lay out a few options to consider:
1. Amazon exclusivity ("Kindle Unlimited" to readers, "KDP Select" to authors) or not? As you are on your first book and marketing from scratch, it's worth some consideration. In Ebooks, Amazon has the largest market share. In exchange for publishing your ebook exclusively on Kindle, in 90-day enrollment increments meaning you can get out in the future, Amazon gives you a couple marketing tools. These include the ability to offer your book for free or at a discount "countdown deal" for I believe 5 or 7 days every 90 day period. Paired with some free or low cost promoting, it can help establish some readers. One strategy might be to start your books in KDP Select until you build readership, then expand out to other outlets.
2. Independent of that decision, Amazon also has "Amazon Marketing Services" at ams.amazon.com. They are still offering $100 worth of free clicks for pay-per-click advertising. Once you are up on Kindle, you can set up a product ad through this portal and bid on clicks. There are a number of blogs out there that explain how to optimize your results. My short version: Use "Sponsored Product" ads, use at least 300 keywords per ad (primarily author names and titles of books that you think are similar to yours as well as more general genre or theme-related words or phrases), bid at whatever level you are comfortable with (I'm typically around $0.25 on a fresh ad for each keyword but competitiveness of bids does depend on genre) then tend your bids like you would a garden, as you can raise or lower them by keyword as needed to get your ad seen.
3. On book covers, I've gone both the custom and pre-made cover route. Just starting out, I see no reason to avoid pre-mades provided you have a good eye for quality. The last vendor I used was Ampersand Book Covers and I bring her up because I had a good experience with the vendor and also her pre-mades have a fair amount of romantic and YA themes. There are a slew of cover artists out there, however, just google "premade book covers". KBoards probably has testimonials/referrals to offer as well.
4. On formatting: It really isn't hard to do yourself. Amazon has published a guide that is easy to follow. I format in Word, upload to the Kindle Direct Publishing portal, and get good quality outcomes. (If you plan to go wider than just Amazon to start, it is easy to format for Nook once you've got your Kindle Word file. After that I do a Smashwords version - Smashwords being a distributor portal to get to iBooks, Scribd, Kobo and elsewhere.) If you don't want to do it yourself, again check Kboards, or if you are looking to distribute widely, consider one of the formatters you can find through Smashwords. While I've never used one, Smashwords keeps a curated list I believe and the pricing has always appeared reasonable.
I hope this helps with some directions you can go. Feel free to ask more specific questions. Mr. Trede just published his eighth book last week, and guess who is his formatting and marketing arm?