Hi there,

just bought the game and learned the basics, im thinking of installing mods soon.

Any recommendations for mods?

What is the best sources to get mods?

Is it possible to download steam mods without the game and install manually?

Best source I've been able to find is steam. Also check this post to download mods directly off of stem.
https://www.gog.com/forum/rimworld/guide_how_to_download_mods_directly_from_steam

First I have to agree, basically all mods for Rimworld are on Steam.
And yes, you will need SteamCMD to download them.

But I really recommend not using it directly.
There is an awesome mod manager called RimPy.

You will find a link in the description here:
https://steamcommunity.com/sharedfiles/filedetails/?id=1847679158

Or you can go directly to:
https://github.com/rimpy-custom/RimPy

You can browse, download and sort your mods comfortably with it.
It internally uses SteamCMD but can be used even without a Steam account (logging in anonymously).

The first mod I'd recommend is the the one from the first link.
It is a meta mod that provides RimPy with a database to better sort mods.

Now for further recommendations, that is very much dependent on your own play style.
From little changes like P-Music for more in game music or RimHUD for a better overview of the informations,
over quality of life mods like Achtung!, which gives you better control over your pawns,
to DLC sized mods like Rimworld of Magic, which introduces classes and RPG elements,
and even complete mod frameworks like the Vanilla Expanded Framework, which has more mods than I could list here.

There is a mod for almost anything (and enough reasons for me to have over 500 of them installed).

-Rimpy is a must and makes getting MODS and dealing with conflicts easy.
-The best mods to start with and the ones which usually have the fewest conflicts are the Vanilla Mods.
-Vanilla Psychic Abilities is a lot of fun, adding piles of cool psychic abilities. These are a lot of fun but sometimes feel a little too powerful. The Mind Control power for instance makes it really easy to capture enemies and disrupt attacks, and the Haywire power makes Mechanoids fight each-other, which is tremendously useful. Tynan generally makes powers with this much impact single use (ie. Psychic Lances) so having a multiple use power can feel a little game-breaking.
-Vanilla weapons include some primitive flintlocks and muskets, ranging all the way up to gauss weapons which punch through enemies to hit targets in a line, and powerful laser weapons.
-Vanilla Traits is a funny one. It adds some cool new dimensions to your characters, but makes recruits a little more random since there are so many more traits that they can acquire. Some require micro-managing and make direct orders for pawns necessary or less effective. Overall they are fun.
-Vanilla textures adds graphical upgrades.
-Multiple Vanilla Animals mods give you way more animal variety in-game.
-Vanilla factions/storytellers change the feel of the game in some pretty cool and interesting ways if you select their storyteller, and just installing them can give you neat themed items, specialized quests and even boss fights against unusual creatures. I found Vanilla Vikings to be a lot of fun. Be aware that some of them, like Vanilla Factions Expanded- Mechanoids makes the game harder.
-Vanilla Events adds variety to in-game events and most really add to the game. I like this one.
-Vanilla Plants includes
-Vanilla apparel adds some great choices for clothing including utility clothing like tool kits, medical scrubs and more, applying useful bonuses for people who wear them.
-The Vanilla cooking and brewing mods allow you to brew cool specialty teas and coffees, grill up ribs and steak, add hearty stews and soups and just gives piles more options for cooking, each with in-game modifiers.
-Vanilla books previously let you read and write books, but since that has been included in the 1.5 update it's probably redundant now.
-Lots more cool ones to look at: trading, spacer furniture, expanded bases and cool scenery.
-Stay away from the Vehicle Framework mod for now. It's pretty buggy, with some cargo loaded onto vehicles just disappearing and issues with helicopters landing.

A large number of mods for rimworld are on Rimbase.

https://rimworldbase.com/rimworld-mods/

Seconding Rimpy, it's a lifesaver!

My (mostly QoL) must haves:
Almost There!
CM Color Coded Mood Bar
Clean Pathfinding 2
Follow Me
Go the F*** to Sleep - Forked
Injured Carry
Interaction Bubbles
Map Preview
Medical Tab
LWM's Deep Storage
Pick Up And Haul
RimHUD
Share The Load
Snap Out!
Where is my weapon?

Other than that all Vanilla Expanded are absolutely top tier.