Converting an 80's Trek to a fixie. This is my first one and I was not sure if i had to get a new bottom bracket for the crankset to fit.

Road or MTB? Got pics?

If it's got horizontal dropouts you can just remove unused chainrings and try to get a good chainline by putting the one you use on whichever side of the spider works best. You may need spacers to get it dialed in. You may wish to consider getting a single-speed specific chainring at some point but I've had good luck with some multi-speed chainrings.

If it has vertical dropouts you'll have to shoot for the magic ratio or go with a tensioner or eccentric BB.

Oops, just reread your post and noticed you said "for the crankset to fit". Thus I assume you're not using the stock cranks. Which crankset are you using? I'd say chances are fairly good you'll need a BB to go with your crankset. I'll put 'em at about 33%, which may be a bit conservative. I'll update probability after knowing more about parts involved.

this is the crankset i want to use. I wish i had pics of my bike but it's an 80s Trek Road bike if that helps any

https://www.bikepartsmart.com/tec9-fi...5mm-170mm.html

Well that crank needs a 103mm JIS bottom bracket. If the Trek has Shimano or Sugino or Sakae cranks then they are JIS and so is the Bottom Bracket unless someone has done some incorrect parts swapping in the past.

Remove the old cranks and measure the BB spindle in millimeters.

I got an ENG. Is that a big problem

If by ENG you mean English threaded BB, that's a good thing. English threads only have to match up with the frame and all Treks have English Threaded bottom bracket shells.

ISO or JIS in this area concerns the taper on the BB spindle. AND you'll need to figure out if your BB spindle is 103mm.

Do you know the model and year of your Trek? Do you know the model of your BB? If not can you take good pics of your BB?

The bike is a Trek 330 from the 80's and the bottom brack is an origin 8 103x68 English. right now i'm having a hard time installing the bottom bracket because i can't get it apart. meaning that it comes packaged screwed together feels like, so i can't get it apart to install.

this is what it looks like

https://www.origin-8.com/?page_id=91&...OTTOM+BRACKETS

On your new BB, one side will pull right off and the other side is pretty much permanently attached.

Screw the side that pulled off into your bike (make sure you're putting it into the correct side of your bike by noting L/R markings on BB body.) If it's the right side (AKA drive side) it will screw in counter clockwise. Don't tighten it down, just get it most of the way in.

Then insert the rest of the BB and tighten.

Then go back to the first bit you put in and tighten it.

There are probably videos that show this being done on Youtube.