You are likely at level 20 because you opted for one of the premade builds. This just allots you all the special stats and perk cards for a specific build but you can change them. You could also roll a new character and opt not to go with a build and you’ll start more traditionally at level 1. Ultimately your call, I’d say as a new player a build might help you a bit more with survivability and access to some helpful perks but you level pretty fast and there is tons of content to tackle.
For repairing, it is a bit different from previous games. Before you would just repair with a similar weapon. In 76 you need to acquire scrap like in Fallout 4. You can find these items around the world and each weapon and armor needs different components. There are perk cards to help with junk weight and finding more components. You’ll frequently find workbenches around the world and can break down all scrap there which also reduces the weight. You can also breakdown weapons which help you acquire plans for new weapon components like better damage, scopes, faster reloads, etc. You can also get repair kits just by playing daily, there’s a free item every day.
For caps that one is a bit tough. Early on you are gonna be a bit more reliant on just finding as much gear and junk as you can and will regularly be swapping out for better items as you level (weapons and gear have tiers based on player level, a level 20 gun does more damage than say a level 5 gun). As you start I’d recommend trying to just scrap everything you can to get new plans and components and stay fresh with repairing components. If you choose to sell you’ll notice things are often not selling for their listed price and so you’ll always be making less. You get decent caps from just completing missions, as well as weapons and gear (including legendaries) and eventually you can build a store in your CAMP to make some extra caps too. You can try visiting other players camps to see what they have for sale too. I try to keep all my prices low, especially for plans since some items are just random RNG drops.