To play old pokemon game you just need a Gameboy, any product labeled "Gameboy" that isn't the Gameboy Micro will suit your purposes. If you need a new one, your best options are the following:
Game Boy Color (not recommended)
There's no reason to get a gameboy pocket (or, barf, an original gameboy) since the GBC can easily emulate the monochrome display if you MUST not have color. Otherwise, the GBC allows gen 2 to be played in proper color as well as lending a less-impressive splash of color to the original games.
Also the GBC can easily trade via link cable to other gen 1 games, assuming you have friends to trade with (pretty unlikely these days).
Game Boy Advance SP (recommended)
Might be a bit more expensive, but the first built-in backlit and has rechargable batteries. Most people forget that old gameboys had disposable batteries and non-backlit screens. Only potential negatives are a possible cost increase compared to older models, and the GBA SP isn't quite as comfortable for extended use--I've only found this to be a problem with "twitch" games like Mega Man, shouldn't be an issue for Pokemon.
There are apparently "universal" link cables that let you attach a GBA to a GBC for trading like gen 1&2 require but I haven't used such hardware.
I know this isn't the question, but take heed that old gen pokemon games really don't hold up as well as you might hope. The PC box system was AWFUL in gen 1 and still poor in gen 2. I highly recommend going for the gen 1 and gen 2 remakes personally, they're available on GBA and DS respectively (FireRed and LeafGreen for gen 1), so a single DS system would be enough to play both of them, and all following generations up to 5.
Yes, but you will need Pokemon Stadium and a transfer pak. A strong team of 5 pokemon and a pikachu on your pokemon yellow cartridge is strongly recommended (because you can't use a rental/registered team on stadium).
Now do the following:
1) Get to round 2 (prime cup, master ball division) using any pokemon.
2) Now enter the tournament and select pikachu and your other 5 pokemon
from yellow.
3) For every fight, select pikachu. You don't have to
use him at all, just have pikachu on your team every round.*
4) After beating the round 2 prime cup** (master ball div) you'll get a
small cutscene before confirming which move to replace with surf.
Note*: Since pikachu is practically useless in the fights, you need to have really good pokemon that can basically 2v3 the opponent.
**Note****: You don't have to be flawless in the tournament, you're allowed to lose a battle
Best Answer
A Critical Hit is an effect applied randomly to an attack which grants it increased damage output. Like a lot of the core mechanics of Pokemon, the way crits work have changed a lot over the years. As you are specifically asking about Generation 1 games (Pokemon Red, Blue Yellow and Green), this is how it works for those games specifically:
Probability
A critical hit occurs randomly. A number is randomly drawn from 0 to 255. This is then compared with a "threshold" value (also 0-255) that is calculated based on the factors below. If the random number is less than the threshold number, the attack is a critical hit.
Because of the way this calculation is performed - all possible values fall between 1/256 and 255/256 chance. Yes, this means it is impossible to guarantee a critical hit in Generation 1 games.
There are multiple different factors that can influence the threshold calculation (and therefore, the chances of a critical hit occurring):
Damage
When a critical hit occurs, the attacking Pokemon's level is doubled during the damage calculation, which (approximately) doubles the damage dealt. Crits ignore all in-battle stat buffs or debuffs, meaning it's possible to land a critical hit that does less damage than a regular hit.
See Bulbapedia's breakdown of the calculation for more info.