In terms of mods, I would recommend downloading, setting up, and taking a look through CKAN. Just search for how to do it on YouTube, trust me it's nothing difficult. Personally I enjoy most mods by Roverdude (Including USI Mods, but also 'Sounding Rockets!') and mods by Nertea (The Near Future and Far Future mods, however these can get complicated to work with very fast so maybe not these). These are two big names in the modding community.
Firstly and most importantly, get the latest version of Module Manager. This is a mod that allows mods to modify parts from the base game or other mods without having to change the actual files manually. It can be complicated but a fun thing to get into nonetheless. It is also required for most mods, so just grab it to make sure you have no dependency issues later on.
But for general Quality of Life mods I would recommend Kerbal Engineer Redux, MechJeb 2, Editor Extensions Redux, and Trajectories.
The first two tell you generally very important information about your craft while in flight and in the VAB/SPH, such as Delta-v, TWR, Biome, Situation, etc. In MechJeb's case it also offers things like autopilot for rovers and planes, automatic course plotters, and other things that get unlocked along the tech tree to help with the progression feel.
Editor Extensions Redux is helpful for building craft as it allows you to get up to 20X symmetry and odd numbers in between like 5 and 7. Another key feature of the mod is that it allows you to Autostrut all parts with a single button.
Trajectories improves upon the base game trajectory map, accounting for atmospheric drag and other minor things that can help you pinpoint your landing site.
Another mod I would recommend if you have many separate part mods would be The Janitor's Closet. This mod allows you to filter parts by mod, helpful for large suites of mods.
Lastly, I would suggest just generally searching for 'KSP Tech Tree Mods'. Community Tech Tree (CTT) is a good one that depends on either other mods to write in compatibility for the tech tree or for you to write Module Manager Patches, and is also compatible with most USI mods.
Otherwise any number of custom tech trees can be very fun to play with.
Hope I helped!