How do supports work?

You build support points by fighting adjacent to units, healing or dancing them, or fighting with them by doubling up. You can activate the conversations from the world map at certain thresholds. Everyone can have unlimited A supports but only a single S support per character, your Avatar can S-rank with anyone of the opposite gender including children.

What the dragontits is up with marriage?

When a first generation unit hits S rank with someone, the two are married. Starting after Chapter 13, this will begin unlocking paralogues that let you recruit second generation units. Their stats are based on the caps of their parents, so a Severa by Gaius will come with extra speed and a Severa by Vaike will come with extra strength. The second generation units all come at level 10 with associated abilities of their base class, as well as an ability from each parent. The ability is the BOTTOM-MOST SKILL that the parent has equipped.

All female characters prior to Lucina have a child associated with them regardless of who they marry. Chrom and Avatar have children specific to them. Pairing them up with one of the aforementioned wives or pairing a female Avatar with Chrom will result in the two children being siblings.

Of additional note, Chrom will be married at the end of Chapter 11 automatically with one of five characters based on who he has the highest support points with, if they're tied, the order goes Sumia, Maribelle, Sully, Olivia, Avatar. If he somehow managed to get here without a single support point with any, or all the wives are taken or dead, he gets married with the random village girl you save in the prologue chapter.

I can't decide on a waifu!

For your first run, just do yourself a favour and don't bother with eugenics. Just pick the wife that catches your fancy and don't worry about stats. If you absolutely can't decide, marrying Lucina gives you a few altered scenes in the story that you don't get otherwise, so she might be good on a first run for the sake of the story.

What asset and flaws should I have?

It depends on a lot of factors, there isn't an absolute best spread and your choice also affects your children's stats in addition to your own, so if you have a waifu in mind already, you may want to tailor your growths around your child. If you're completely indecisive, Resistance/Luck gives you the most balanced stat spread in the game, with only HP and speed going over 50% in growth, and nothing going under 40%.

When should I reclass?

The ability to reset your level means that promoting early doesn't lose you out on much, it's recommended that you do so and focus mostly on the skills you want to pick up. Due to how the internal experience system is managed, it's recommended to promote, hit the levels you need to get what you want, and then reclass to a tier 1 and promote from there into other classes. Exceptions can be made where ever you want them to be made though.

What class i-

Sorcerers.

I got a bonus map from Spotpass, but I can't access it, why?

Spotpass maps such as The Dead King's Lament can only be accessed once you reach the Endgame(after Chapter 25).

I started a child's paralogue but their stats and skills aren't what I wanted. Am I screwed?

No, you can just exit the map and rearrange/learn new skills, and grind a bit to raise the parent's stats. When you reenter the paralogue, the child's stats will be updated with the new parental stats and skills.

Useful Links:

Eugenics Planner
http://www.serenesforest.net/fe13/char_growth_all.html