I just bought a 100 amp breaker panel to replace an old fuse panel. The thing is, after I figured up everything I wanted in the shop, (21 receps. 10 lights, 8 switches{4-3way switches and 4-single}, 3 fans{2 w/ lights}, and 3-(240v)plugs), I'm now not sure if I can pull all them on a 100 amp panel. And how would I figure that up to know. I really need to know soon. Thanks, Nate!!

The only thing you mention that has any draw is the lights and fans. Even they are not that big of a load.
Switches do not draw anything.
Receptacles only have a draw when something is plugged in, even the 240v ones. ;)

The only accurate way is to do a demand load calculation. Not the simplest thing for a DIY.
If this is for a house then ALL the general use receptacles and lighting are taken into account in the load calc. The actual number of them is meaningless.

I can tell you from experience, what you have is FINE on a 100A panel.

Your layout process is out of whack.

To begin with, this 150 Amp sub-panel requires a four wire feed from the main panel.

If the sub-panel is going to be in a different building from the main service panel, then you must also install a complete grounding system at the sub-panel.

You will also have to isolate the Neutral Bus from the Ground bus and Neutral cannot make any contact with the sub-panel cabinet.

It is also mandatory that if you run power to an attached building, you must put a light outside the entry door.

The required amperage is determined by calculating the required load, not by counting up devices you think you may need. Also, receptacles in a garage or shop need to be GFCI protected either with a GFCI receptacle or a GFCI breaker protecting the entire circuit.

This is not a simple calculation to do so I recommend that you get at least three licensed electricians to bid on this job.

How many circuits are in the new panel? I only see you needing about 7 or 8 circuits. You have to think about how your are going to be using your tools. Are you only one person? You can have three table saws, two drill presses, a lathe and a band saw, but unless you are faster than I am you will only use them one at a time. What kind of fans, ceiling fans with light kits. Compressor, dust collector?

What is size of breaker in main panel? What is size of present fuses box?

I'm out of whack! Why?

Are you saying that you are running a new service to this building, because if you are, than you left that out of your initial post. Maybe you should provide decent information on what you are trying to do rather then leave everything open ended.

But regardless of that, you would should still have do a load calculation to make sure you have the correct breaker or main cut-off installed.

If you do not understand what I wrote or want more information on why I wrote it, how about asking me before you slam me.