My first time playing D&D. I’m starting with the default equipment my class gives me (I’m a rogue), so my question is: how much money would I have? I know I can roll 4d4 and buy the equipment and I did that but ended up with 54 gp. Is this too much? If so please explain how I do this.
There are exactly two ways to determine starting equipment.
Take the starting equipment listed in your class, and the starting equipment listed in your background. The latter will detail how much (not much) gold you'll start with.
This is the method you described, taken from PHB p. 143 - You roll 3 to 5 1d4's and multiply it by 10gp, with the number depending on your class (Monks also don't multiply it, since they don't need starting weapons or armor). Obviously, this roll should be done with your GM present.
Which option is superior? That's ultimately up to you and how much you like randomness. Keep in mind that some GMs may disallow one method or the other, and in fact, the latter method isn't allowed under Adventurers League rules (though I hear you may sell and buy equipment before your first adventure but that's another question).
Money past your starting equipment comes from your background (PHB pp.125-141).
As an alternative, you can use the alternate rule found on PHB p.143 to eschew your class's and background's starting equipment, roll for coin, and kit yourself out as you see fit.
So to be super-specific in your case: you can have the rogue's default equipment and 0-25 gp from background, or roll and end up with 4d4x10 gp in equipment and coin.
You have a choice between two standard methods for determining your starting gold. Beyond that, you need to ask your DM.
The two standard methods
Chapter 5 of the PHB, at p. 143, gives you a choice of two options for your starting gold:
When you create your character, you receive equipment based on a combination of your class and background.
If you look through the classes in Chapter 3, you'll see that each has a standard set of starting equipment choices. Similarly, all of the backgrounds in Chapter 4 have standard starting equipment and at least some amount of starting gold.* If you take this option, you get that combination of equipment and gold to start with.
Alternatively, you can start with a number of gold pieces based on your class and spend them on items from the lists in this chapter. See the Starting Wealth by Class table to determine how much gold you have to spend.
This option involves rolling a predetermined set of dice, according to your class. The table mentioned in the above quote lists the starting gold for a rogue as 4d4 x 10. That's 4 four-sided dice, times ten. If you take this option, you roll those dice and use the result as a pool of gold with which to purchase equipment. Chapter 5 lists the prices of all the available equipment. When you're done purchasing equipment, the remainder of the gold you rolled becomes your starting gold (and silver and copper, to the extent you have smaller change left over).
There shouldn't be any mystery as to whether the 54gp you're saying you've wound up with is "too much." It's just a matter of math, depending on what the dice result was and what specific equipment you purchased with the resulting gold. If you did the math correctly, and 54gp is the result, then no, it's not too much.
An additional, optional source of starting gold from XGtE
There is an entirely optional set of additional, potentially relevant materials starting on p. 61 of Xanathar's Guide to Everything, called This Is Your Life. TIYL is a set of tables intended for generating character backstories, including "life events." Some of the entries on those tables include gaining a small amount of gold or equipment (such as trinkets or potions of healing).
Note that the TIYL material is explicitly "meant to augment" the standard character creation process as it appears in the PHB. In other words, any gold or equipment gains from using the TIYL tables are in addition to the standard methods for determining your starting gold and equipment.
Nevertheless, I stress that TIYL is entirely optional. It is presented as "ideas, not rules," and that relatively loose structure might not be a fit for the game your DM wants to run, nor for the game your fellow players want to play. If you're interested in using it, you need to check with your DM.
*Fun fact: not all official WOTC-published backgrounds come with starting gold. All of the ones in the PHB do, however -- as do the ones in Sword Coast Adventurer's Guide.
You have to choose. You can either take the default start equipment, or roll and get the cash and buy starting equipment:
When you create your character, you receive equipment based on a combination of your class and background. Alternatively, you can start with a number of gold pieces based on your class and spend them on items. See the Starting Wealth by Class table to determine how much gold you have to spend.
If you're saying you bought equipment and now have 54gp left, I can't check the maths unless you say what equipment you bought.
Your background also provides you with equipment and gold, only if you chose to not roll for gold.
For example, if you were a criminal, you'd have:
A crowbar, a set of dark common clothes including a hood, and a belt pouch containing 15 gp