Alright, now I know this is disappointing as far as first tips go, but this one is pretty important. You need to, well, know what's important.
Prioritise which weapons and gear you really need. Having passive gear buffs on each equipment slot is important, but do you need every weapon? Are you going to use them all?
Take a good look at the stats and judge for yourself which best suits your playstyle and dedicate yourself to the choice, before being frivolous with all your cash.
Mission on, level up
You need cash, and the best way to do is to clear missions. Yeah, it's a bit like a job.
Go to Mission mode and take a good look at the missions you have available to you, many of them will relate to specific weapons, hitting certain scores, or getting a number of kills.
Most importantly, you should feel free to just skip Missions which seem unreasonable to complete or awkward. You'll have to watch an advert, but the short wait time will be worth the rewards.
Taking it online
Jumping into online games will certainly change things. Online players will be at different levels, using weapons you might not be used to, and using tactics you can't prepare for.
But that doesn't mean there aren't rewards to find. You can make a lot of cash by killing enemies and winning games, and it's a nice change of pace from the Mission mode.
Whether you want to go in solo or with a team, make sure to keep your back covered, and remember that shooting from above will often get the jump on foes.
Gotta spend money to make money
Okay so this is a bit counter-productive to our first tip here, but if you're following all the tips so far, you should be getting a good amount of cash stocked.
Yes, you'll need to spend money to make money. You can complete more Missions with new weapons, as Missions will appear pertaining to that weapon.
The same applies to gadgets, meaning you can can clear more missions if you have a greater variety of artillery at your disposal.