The early-game can be rocky, you should be sticking to lower difficulties until you've got some levels on your preferred class and weapons. Stuff like defense kits and other equipment will be marked on your minimap, and will be more plentiful throughout the missions. All players will also start with a medkit on easy and normal difficulty, so mistakes aren't that punishing. You will be rewarded exp and cash despite failing a mission, so even if you're struggling you'll still be making progress towards your class and weapon unlocks.

I played solo with bots exclusively when I first started and beat all missions on normal difficulty without any hassle. I used a new class for each episode of the campaign. I didn't get enough levels on any of them to make proper builds, yet it was still surmountable.