A very good place to start is forming good defensive habits as early as possible. I'm the only huge fighting game nerd in my area, and the few friends that I have know me real well for setting up some different, random FG every time a bunch of people are in the same house.
I'm barely an intermediate, even at my best game (which is SG), but I'm still the best in our group, so I've played a lot with people WAY below my skill level. It's extremely common for low level players to either smash lots of random buttons or drop the controller once they lose neutral, both of which are basically just suicidal.
Fighting games are odd, because they kind of ask you to earn your health bar. When players without a lot of experience complain about being stunlocked or trapped in infinites, this is usually what they're talking about. Getting hit puts you in a disadvantage state, and there aren't a TON of things that will get you out of it.
It's a bit like a complex game of Rock/Paper/Scissors, where not throwing a hand means you lose by default. It's easy for a player with just a half-ounce of skill to loop some basic attack string until you're dead if you don't know how to defend yourself, which means you might as well be playing on a magic pixel the whole time. I can give a bit of direction for this in SG.
Practice your reaction time. The tutorial tells you to crouch block by default, and stand up when you see an overhead or jump. This is good advice. You can practice this by going into Training Mode and using the Dummy Recording setting. Record something like a dash into a low attack in Slot 1, a jumping attack in Slot 2, then set playback to random. Being able to do that consistently will help you a lot.
If you're consistent at that, try something like Squigly c. LK > c. MK in Slot 1, and c. LK > Forward + HP in Slot 2, or Big Band c. LK (wait) c. LK in Slot 1 and c. LK > Giant Step in Slot 2.
Practice this stuff for a couple of minutes to warm up before playing the game. If you get frustrated on a losing streak, take a break to cool off. Being hot-headed makes it hard to focus, and easy to start making dumb decisions or react too late. Practice something like this in Training Mode for a bit before going back.
If you don't know what to do, hold a random diagonal direction and pray. Not all mix-ups can be avoided on reaction. And if your reaction time is very slow, stuff that would be easy for somebody else to escape might be invisible to you. So if reacting won't work, and they're not being predictable enough for you to already know the answer, just make a guess and hope for the best.
Holding Down and Back will avoid Low attacks and anti-air grabs, but lose to throws, overheads, and side-switches. Holding Up and Back will avoid throws and overheads, but lose to Low attacks, side-switches, and anti-air grabs. Holding Up and Forward will avoid throws and side-switches, but will lose to a well-timed Low attack, overhead, or anti-air grab.
Just guessing wildly will only be right a little less than half the time, but giving up or mashing buttons will be right never. It's better to guess than just give them free damage. Remember that even tournament champions block stuff wrong all the time. You don't have to be perfect, you just have to TRY.
Remember to pushblock a lot,. Pressing two punch buttons at the same time while you're blockstunned will push your opponent away from you and make it harder for them to keep you locked down. This is crucial to getting out of a disadvantage state. It can be baited and punished if you're super predictable, so you can't do it every single time. But you have to make them worry about pushblocking, or they'll just run you over. Remember to actually do this sometimes; it's important.
Consider not ground teching sometimes. If you get knocked off your feet and crash into the floor, you can do an invincible roll to get back up and choose whether to move forward or back when you do. This empties half of your Undizzy meter, which determines how long your opponent's combo can be if they hit you. And if Undizzy is full, I'm pretty sure all hits always do at most 55% of their usual damage.
So if you ground tech and screw up blocking, you might eat half of a full combo for big damage. If you don't tech and block wrong, then you'll only eat a few hits for half their usual damage before you can guess again. If you just have to guess a lot anyway, you might as well give yourself more chances to guess.
This stuff applies to basically every single character in basically every single matchup. This can help improve how long you're able to survive, and maybe sometimes get out of disadvantage and back to a place where you can play the game again. without just dying every time you make a simple mistake.
Improving your combo damage can help you win by requiring you to win neutral fewer times, but the advice for that is extremely specific to which character you're playing. If we know what character(s) you're playing, we can help you find easy combo strings and help you troubleshoot problems you're having with execution. Don't worry about doing full length BnB combos or damage optimal stuff yet. Learn something simple and work your way up.
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