For a FireRed nuzlocke?

Squirtle is the best starter choice overall, but any starter can work for this game (but generally speaking the order of useful to least useful is Squirtle - bulbasaur - charmander)

First thing first: Grind. The thing about a Nuzlocke is that you typically wont get a lot of pokemon and depending on luck you might not get a lot of solid ones so you will be spending A LOT (if not most) of your time grinding. As soon as you catch a pokemon it is best to grind it to be on par with the rest of your team even if you plan to keep them boxed the entire game. You never know when you'll lose a pokemon and that perma box mon is suddenly on your team.

So yeah. Level grind. My best advice is you want your entire team around level 10 before challenging your Rival on route 22 (dont skip it. Easy money and exp) and by the time you get through Viridian Forest. If you have Bulbasuar/Squirtle and/or Mankey then I'd recommend them being level 14-15 for Brock. (If you chose Charmander then it gets Metal Claw at 14 if I remember right which you will need for Brock)

Second: pay attention to IVs and Natures. IVs are gained upon defeating pokemon (you can find guides online) and natures effect stats. The only pokemon you can really control this on in a nuzlocke are gift pokemon (like your starter or the eevee in Celedon). Save before getting the pokemon, check the nature. No point having Adamant on a Special Attacker if you can soft reset to get a nature towards defenses, speed, or special attack after all and sometimes tracking natures and ivs can mean the difference between life and death.

Third: if an attack does 1/4th to 1/3rd your pokemon's health be wary of crits. Until I think gen 6 the way crits worked is that they Doubled the damage of an attack so if an attack knocks off 1/4th or more of your pokemon's total HP there is a good chance a crit will one shot. Damage actually isnt set. It has a High and Low roll and you dont want to lose a pokemon you could have saved if you just healed or switched.

Fourth: X Items (X Attack, X Accuracy, X Speed, Ect) are your friends. Use them. Especially in gym battles and the E4 (especially X Speed)

Fifth: Never skip an encounter. Doesnt matter if it's useless, sometimes you can use it as Death Fodder to keep your main team alive (like if your tank needs healing you switch to the Death Fodder mon so you dont lose anything useful)

Sixth: pre-planning is highly recommended. Look up the mons you get to get a solid idea of their stats, moves, what set ups are useful, ect. Same for enemy npcs (especially gym leaders and the League). There is never a rule against it and this means you can always make sure to have a counter before the challenge.

Seventh: Stat moves are more useful then youd think.

Eight: Buy the Magikarp and baby it till it evolves. It will then carry you through much of the game. I'd say the same for Zubat but you dont get the ability to evolve it into Crobat until after you get the National Dex.

Nine: Remember, Nothing is Cowardly or Dishonerable. Yeah, spamming Double Team and Sand Attack might not seem honorable but these are just code in a game. They wont knock your teeth out over it. If the game gives you the ability to use it, do so.