mono red is very much the done thing in pauper. interestingly, I havent encountered a single rat deck yet, and I've been playing for a few days.
Regarding Sealed, I think it's important to keep in mind that limited as a format requires a slightly different skill set to regular constructed play. So, if you get hosed the first time you try, don't feel too badly about it. After all, at least you get to keep the cards, aye?
Advice-wise, there are a few key things that really matter.
1. Know the set. By which I mean, know the most important mechanics, and actually know pretty much all the commons and uncommons, as well as all the important rares. Obviously this is a big ask, so as an alternative, spend some time reading this page (and the linked ones for the other guilds/colours.) https://www.channelfireball.com/article ... w-azorius/
Luis Scott Vargas (LSV) is considered to be a bit of an authority on Limited play. His reviews are solid, and easy to follow. He's mostly writing for draft, but his points are equally valid for sealed, and he'll usually say if there's an exception to that. I would strongly recommend using his card reviews while you're building your deck. You have all the time in the world when you're picking cards in Arena, so use it.
2. In Allegiance, colour pairs matter (in other sets, not so much.) I'd strongly recommend picking a guild colour pair and not deviating from it unless you see an absolute doozy of a card. Even then, don't splash too far into a third colour, and certainly don't splash for a card that needs 2 of a colour outside your pair (eg if your chosen main colours are Red Black (RB), you could possibly include an especially good Red White or Black Green card, but not a BlackBlack card or a BlueWhite card.) If you do splash a third colour, consider including some kind of mana fixing in your deck (eg an artifact that can make any-colour mana, or a Gateway Plaza land, etc.)
3. Try to have some kind of synergy/game plan for your deck. eg try to make cards give you more than they do by themselves. The key things to look for in Allegiance are:
Rakdos: Spectacle - this makes cards very efficient. Eg Skewer the Critics is a sorcery that deals 3 damage to any target, costing 2R mana, but changes to R (casting cost) if you've already dealt damage to an opponent this turn. 3 damage for 3 mana is bleh, but 3 damage for 1 mana is awesomesauce.
Orzhov: afterlife - turns your dead dudes into 1/1 flying spirits. This can work well with other cards that make you sacrifice your own guys for some kind of benefit (cos you still get to keep the wee spirits afterwards.) Or just cast Ethereal Absolution and watch your opponent quit.
Gruul: Riot - makes your beasties bigger/faster. This is a very good mechanic. Rhythm of the Wild is superb, and if you get one of these and have decent gruul cards, yer in. When your creatures Riot twice (from their own card, then again with Rhythm) it gets busted, fast.
Azorius: Addendum - adds a (small) extra effect if cast in your own main phase. honestly, it's a bit underwhelming. There are some excellent Azorius cards, but their special rule is underwhelming.
Simic: Adapt - puts +1/+1 counters on creatures for an added cost. This is a very good mechanic in Limited, as your low cost creatures (that you can cast early) can become larger threats later, as there is less removal in this format so they're likely to stick around long enough to be useful. Also, in limited you sometimes find yourself with mana available but no spells to cast. Soak up that mana by Adapting your dudes. Anything that flies and has adapt is a good card. Or Hydroid Krasis
4. Limited formats (draft and sealed) tend to play slower and have lower overall power levels than constructed, since you have a smaller pool of cards to build with, and will almost certainly not have more than 1 of any given rare. As a result, big fat-ass creatures that cost a lot of mana are actually pretty viable strategies. Those 6 or 7 mana green creatures that you'd normally not have a chance to play in decks become a serious threat, as most games will go to turn 7 or more in limited. In fact, creatures are almost always how games are won in limited. You just cant pull off the fancy sorcery/instant combos you would in a normal deck. So, factor that into your deck building. Make sure to have plenty of creatures, on a nice curve from 1 mana to 5 or 6 or even more if appropriate. You want about 17 creatures and 6 non-creature spells, with 17 land to complete your deck. You'd need a really good reason to deviate from that ratio.
5. As an addendum to the above, limited games can often turn into stalemates, where both players have 5 or 6 creatures on the board, but nobody can attack because their dudes will get killed then the player will die on the return swing. Always try to have some flyers in your deck, as they can consistently swing in for 2 or even 1 damage when the board is locked like that. It can make all the difference. of course, if you can build a deck mostly around flyers, all to the better. If you dont have flyers, trample or menace will do. Or maybe a spell that stops opposing creatures from blocking, or taps them or summat. something to break the deadlock.
6. As a further addendum to the above, always, always have a few kill-spells in your deck. Either direct damage (eg Shock, Skewer the critics), or kinda-direct damage (eg rabid bite) or straight up kill (eg murder). Sooner or later your opponent will play a nasty game-winner, and you'll want to have something to deal with it. White can also use lock cards like Luminous Bonds to similar effect.
7. It's more important to have a nice mana curve than a killer card in your deck. A well balanced, nicely curved deck will win against a crappy deck with a bomb rare in it 9 times out of 10. Realistically, you should have a bomb or two in your deck anyway (because otherwise why are you playing those colours?) but worst case scenario, go for balance.
8. As an addendum to point 7, you'll find that creatures that are basically kinda meh become useful additions to your deck. Something like a 2/2 for 2 mana, which would usually be not good enough to play, becomes a decent filler. Obviously not all 2 mana creatures are created equal, but if you need to include something a bit mediocre to round out your curve, it's not a terrible thing.
Hope that helps