So I'm fairly new to this, but what's the best way to avoid overheating?
The obvious temporary solution is to put all heat generators on one side of the base, and all heat-sensitive objects on the other. Don't have 10 small batteries and a coal generator blasting heat into the air, directly inside your mealwood farm. It's obvious but i've seen it happen before.
Keep a wider margin between your base and the neighboring high temperature biomes. Remember that mining a tile destroys half its mass. 1800 kilograms of sandstone takes much longer to heat up than 2kg of oxygen, and the 900kg of sandstone that's now on the floor only really interacts with the floor's temperature, not so much the air that's around it. Realise that raw metal will conduct heat faster than oxygen can, so having a large copper deposit touching the hot caustic jungle is a bad idea, and it would be preferable to mine it out immediately. The ideal place to put your heat generators is at the top of the starting biome, and the farms beneath the pools of water or near the center of the biome.
Heat rises and cold sinks. Don't dig too far down until you can get insulated tiles between your living and farming spaces, and the rest of the asteroid beneath you. Keep in mind that every single tile will perform heat calculations based on its neighbor, with a bias towards rising or falling based on its surrounding relative temperatures. If there's cold above hot, the materials adjust each others temperatures much faster than if it was hot above cold. Similarly, a series of tiles that look like "Oxygen - Insulated Tile - Oxygen" is only slowing down the heat transfer by 1 insulated tile math calculation; 2 insulated tiles are much better because of this. (a width of 3 tiles
of any kind prevent liquid from breaking through due to over-pressure; 2 insulated and 1 regular tile surrouding a 10,000kg water tank won't ever rupture and will be very thermally insulated.)
If you absolutely can not research insulated wall tiles in time (due to finding an active steam geyser point of interest) build a vacuum wall to keep the heat at bay, which is done by removing an orthogonally surrounded tile through exploiting diagonal deconstruction:
Numbers as build orders for tiles, periods are spaces:
.2.
212 < After all 5 tiles are built, remove the 1
.2. that tile is now vacuum
Double-wide vacuum space flooring:
5665665 < top of the horizontal vacuum floor
4334334 < remove 4's when 5's above are built
1221221 < remove 2's when 3's above are built
1111111 < bottom of the horizontal vacuum floor
You can shrink the vacuum space to just 1 tile if you want to, but i find this method is generally faster to build or change due to more tiles being accessible diagonally.
Vertical vacuum wall:
DEFG < remove F when tile above it is built
9ABC < remove A when E above is built
5678 < remove 7 when B above is built
1234 < remove 2 when 6 above is built, etc
1111 < starting floor
obviously just rotate the build order 180o for building downwards
You can even integrate a water-lock into these designs to prevent gas infiltration and do work inside the vacuum... HOWEVER, do not send a flatulent dupe to do these jobs, or your vacuum will be filled with natural gas, destroying the perfect insulation the vacuum provides. Don't send a slow dupe either, or they won't ever finish the job. The tiles don't need to be insulation, but if you decide to change the tile type you'll have to rebuild the vacuum tiles. Remember: Any orthogonally connected tiles will transfer their heat to the other -- Diagonal tiles don't.
edit - The ultimate solution to heating problems, however, is to research automation, liquid storage, and the aquatuner, then setup a cold-water cycling pipe in your base to keep your dupes comfortable.
As a side note however:
Never ever use the special thermal-adjustment clothing, as they don't work as you would expect. Sweaters increase insulation which means they take MORE time to change temperature (even standing in liquids!) but they do still change temperature. Cool vests
remove insulation on a dupe which means they'll immediately become hypothermic or heat-stroked if they step in a puddle for too long or stand near an active space heater.