More upgrades, more money
After each level, the game will take you into the shop menu screen - for whatever reason. Here you will be able to purchase upgrades and ingredients.
The better the ingredients, the more money you'll be able to make from each customer, and you'll also be able to store more food, prepare more drinks, etc.
Being able to make two cups of coffee at once makes a huge difference in the early game, and it won't be long until you're unable to settle without all of the upgrades.
It is very tempting to use gems for practically everything, but you must know right now: don't. Gems are precious premium currency, and you'll only be earning a limited amount.
Reserve gems for upgrading your kitchen equipment - upgrading your cookies stores and other things can be seen as a task saved for when you really need it.
Upgrading kitchen equipment will be the primary way to make more money and move through the game, and each upgrade will require some gem investment, so they should be saved for that and that alone.
The juggling act
In the early levels everything should be straightforward and simple. You prepare ingredients, take them off the grill, add toppings, serve to customers, repeat. But it quickly becomes complicated.
You'll be dealing with many customers, and often they'll be asking for three items each, meaning you often will only be able to satisfy a single customer at a time. But at other times, things are slow, and people will only order a single item.
You need to find a balance. When you see many customers ordering a food item, load up the grill constantly, fill all the plates, then slow once things ease to avoid burning things on the grill. If you do end up burning something, tap it three times to trash it - but this should be a last resort.
More to cook
Once you become a master of making pancakes in the early part of the game, Mr. Krabbs will call you into the Krabby Patty to get back behind the counter as the health inspector is coming over. Nightmare.
But this is just the first of several contrived reasons for Spongebob to move through to new areas and find new things to cook - only, your upgrades won't be coming with you.
If you want to get through the game as fast as possible, only get the kitchen upgrades you need to clear levels, and save currency for the kitchen upgrades on the next level, and so on.