$0.02 from someone who went from using WordPress (and constantly being frustrated with it) to Blogger/Blogspot and absolutely loving it...
If you're just getting started, want to pump out the writing, and you're not too finicky about how your your serial looks then WordPress is where it's at. WordPress.com, I mean. It's quick, it's easy. You select a really bad template, do some MySpace level customization within -SEVERE- limits, and you're set. Monkey writing Shakespeare.
If you actually care what your site looks like, know a little bit of HTML/CSS and don't want to spend $300 a year just to be able to add basic CSS functionality to a web serial... Blogger is where it's at. Yes, Google's systems with it are a little strange to get used to. The perks are that you have almost absolute control over how your site looks as long as you're a little savvy with coding and graphic design. You can build your own templates on how things work. You have full access to syncing your fiction up with things like Patreon, Paypal donations, G+ communities, Twitter and all the rest. (FYI, if you have a WordPress.com account, expect to shell out money just to be able to link to your Patreon or PayPal for donations or support. WordPress will -actively- try to shut your stuff down and screw up your code even if you find a work-around.)
If you're "Super-Pro," "Published-Pro" or "Rich-as-F**K", you've already got a webhost provider and you can use WordPress.org. You have full functionality and can do as you please. Must be nice to actually get money for your writing or be the elite 1% lizard-people ;P
When I first started, I absolutely loved WordPress.com. I didn't want to sacrifice the $bling$ on web-hosting for a serial I didn't know if I would commit to or not. I didn't want to have to pay for something I probably would never see a single red cent for anyway. WordPress.com was awesome for that.
Then it wasn't.
You see, WordPress is really hard-up about limiting user's functionality and forcing them with cattle-prods hooked up to car batteries with the limiters off to pay them ridiculous amounts of money just to be able to use anything other than middle-school level usage of HTML. I saw this over the last two years as functionality with WP dropped. CSS is verboten! Patreon, verboten! Changing the god-awful colors on your template that WP keeps changing around on you every few months, nein!!
...And don't even get me started on the bi-polar updates every quarter that totally screw with templates, your font choices, or even the basic layout of your page. I'm sure there's some troll sitting in their server room watching your web serial with glee, ready to attack you with his ultra-sadism +1.
That is when I moved over to Blogger. You know that part in Plato's 'the Cave' where the guy gets out of the cave and sees the real world rather than the shadow-play world? Yeah, it was like that.
I'll go back to WordPress(.org) if I can continue my sacrifices to the Dark Lords and earn my elite status as one of the Setitians. Until then, I'm very fond of my Blogger account. I need to work on my hissing.
As far as going back to the OP's question...
Log in to Blogger.
Go to your Blogger page.
Select a decent Template under your Template section on the left scroll bar.
You'll see little mobile and web versions of your site.
Go to the little orange icon that says "Customize."
This gives you a bit more of the 'expert' level customization values. But if you're savvy and want custom CSS or HTML, go to where it says "Advanced" on the left. Now scroll down the little scroll bar that pops up until you get to where it says "Add CSS" at the bottom. Click that.
Now you can 'top-load' as much CSS into your site as you want. Just make sure to make all the CSS 'conditional.' Meaning to put proper tags on it so you can trigger them in the base HTML on your pages. That's if you want specialized things for special pages or anything like that. Remember that everything you want in the 'header' of your site needs to go here.
To get custom HTML on individual pages/posts, just go to where you make your Pages/Posts under those on the main part of your blog. Then select "HTML" rather than "Compose."
Now for getting tabs on your page, you should already have some basic ones with a basic template you set up. Those will appear between your header image and the main content of your site (usually). You can either visit your page as it is, once logged in and click that little set of wrenches beside the tab bar, or...
On your main Blogger edit page, go to where it says "Layout" now scroll down over the gadget-forms of your page to where you see the gadget "Navigation" which is usually tucked in the area called "Cross Column" on a default template. Click the little blue word that says "edit." A new dialog screen will pop up called "Configure Page List." This is where you add pages as tabs on that navigation bar.
You'll have to create the pages or edit the basic template pages to add them to that nav bar.
To make new pages, go back to the main Blogger edit page and select "Page" make sure to name it correctly and code it correctly then you can add it to the tab navigation menu.
Or, if you're great with CSS, manually program your own. Be prepared to lose hours, days and most likely months of your life. It's fun like that.
Hopefully some measure of help comes from this. Otherwise, look up videos you YouTube, there's a lot of good ones that go step-by-step through it all.