There are several ways to get caps.
1. Private mine + 2 slaves + crafting = a lot of expensive items with bonuses which you can trade to players or npc vendors.
2. Farming encounters and selling to npc vendors then buying items which you can sell in other vendor which has caps.
3. Farming Gun Runners' quest about Mutants' hideout. You can easily solo this quest in turn based, in real time combat too but I prefer tb for it.
4. Lockers in Reno, Warehouse, Glow, V15, Ares. Glow and V15 are easy, you can find useful stuff there which you can sell to players. Reno, Warehouse and Ares are harder but if you really want to you can practice 'soloing' in
private server - generally it is good way to test some things and features.
5. Some repeatable tasks which give you caps - however really small ammount so its better doing caravans instead.
6. Learn mapper and design maps for
mapper contest. If your maps are great, you will get rewards.
Actually, I guess thats all. Maybe someone experienced like Seki might give you some advices too. I do not talk about PvP because it requires more experience and characters to do.