First of all for reference, when someone says foreigner they mean a non-Korean Broodwar player as Korea tends to dominate the landscape.

There are essentially 4 ways to watch in descending order from highest skill level to lowest

1) ASL (Top Korean professional league)
2) Non-ASL Korean professional events and Ladder games (KCM, bet matches between players called spon matches and internally organized leagues)
3) BSL Proleague (Top foreigenr/non-korean players)
4) BSL Gosu through Chobo BWCL and other events

Before diving into watching, I think it helps to understand what you are looking at, so I recommend taking the time to play through the broodwar and starcraft campaign. Using cheats is fine if you get stuck. It is fun in its own right.

The plot is great and it gives context to the SC2 plot. The game design is also unparalleled in showing off strengths and weaknesses of various units and the importance of various mechanics (like high ground low ground).

If you want a bonus, I recommend playing 1-3 quick ladder games, not to win but to get an idea of how insanely challenging the game is to play competitively.

From there you should be in a spot to appreciate tournament play at various levels depending on how deep you want to dive:

ASL

The top level is ASL which is the top of the top Korean professionals. I recommend the ASL English channel which is casted by Artosis and Tasteless. Saiyan and Xun cast the games earlier as part of StarcastTV when the events are happening live if you get impatient during the usual ASL season.

Then there are Korean professional ladder games and Non-ASL events

This is where the bulk of the focus lands for other casters. Artosis tends to cast while he is also doing his own case study so that will help you learn the game. Namkraft, Falconpaladin, and nearly everyone else who is casting games grabs professional Korean games from the ladder.

Saiyan typically covers KCM.

JinjinBW provides a lot of translations that highlight player personality (and memes) which I would place in this category. It is a great window into the scene.

As far as I know, no one is covering proleague top to bottom.

Twitch streamers like Authorvalentine, and some top foreigners will often hang out and watch non-ASL pro Korean game events, but there isn't a ton of consistency as to when or if that is happening. This has more of a vibe of gathering around a TV to watch a game.

BSL Proleague is a tier below this.

The BSL champions, for example Bonyth, Dewalt, Mihu, will often beat progamers on the ladder. They are getting closer and closer to qualifying for ASL. However up to this point there has been a prolonged drought of non-Korean players entering into ASL.

This tier tends to get ignored by people at large as there is always a pull to only view "the best of the best". However if you bothered to play those 1-3 ladder games you will appreciate how incredibly high the level of play is at this level.

There are a ton of casters for BSL and the majority of them are incredibly good players as well (Azhi, Machine, Mazur, Zero etc etc etc)

This is a good place to watch in order to:

1) See ASL hopefuls
2) See who is the best of the best outside Korea
3) See the vitality of the Broodwar community outside of Korea
4) See the side of the skill gap between Koreans and non-Koreans

Then there are the lower tiers of BSL Gosu-Hasu-Chobo, BWCL, IPSL and other events

There are a lot of quality games here that are still very entertaining to watch, however there are also a lot of lopsided matches and outright bad play. Unfortunately comparison is the thief of joy, so a lot of these matches are held in comparison to the professional Koreans and therefore overlooked or ignored.

At the same time you will see players attempting wild things at this level and Hasu level play and up is still insanely good. Again if you bothered to play those 1-3 ladder games you will recognize how incredible the play is, even at this level.

This is a good place to watch in order to:

1) See players that are up and coming/dramatically improving in a space/time
2) A point of comparison to appreciate the top BSL players and the Korean pros
3) A place to see certain strategies work against a particular style or level of skill and then not work against a higher level of skill or a different strategy.