So, for my campaign the premise is that there is no magic no nothing until this crystal breaks and you get your race, magic, your extra damage all that stuff. However, there are no monsters like goblins or anything but there are evil people with powerful magic, I'm not 100% sure how to make fightable characters like that and would like some help.

I'm also pretty new to DM'ing if you couldn't tell.

There are plenty of "monsters" (D&D  monsters can be ant NPC) that can be any humanoid race (if you select the humanoid monsters in DnDBeyond you will find plent of options from bandits and acolytes to Archmage.

You can also reskin others, for example a PC rogue can disengage and hide as bonus actions and many can use a scimitar so if the party are raiding a den of thieves you could have some of the enemies have the stats of a goblin from the basic rules but be human (or elf etc)

Once you get a better handle of different "monsters" you can start creating your own.

I suggest not making it more difficult on yourself than it needs to be: just decide on an amount of hp, an ac, and some spells/abilities for them. You might even just pull features that you think find fit the archetype you are going for from classes, monsters, or just make them up. 

Thanks for the suggestions, I was really stuck on this but now I think I got this, thanks again :)

There are pre-made "monsters" that are things like cultist, warrior, mage. You can always use those.

Also, remember two big things, which are kind of like what above posters are saying. 1. NPCs and monsters don't have to follow PC rules when you make them. You want them to do something, they can do it. You don't have to justify why the enemy who seems a lot like a fighter can also toss around what amounts to bardic inspiration, they just can. 2. Re-skinning is your friend. There's no goblins, that's cool. But you can easily use the goblin stat block, and just say they're human. Or the orc stat block. Or the zombie stat block. That's often the best choice, since you have a handle on how powerful they'll be. CR is imperfect, but it can get you a pretty close idea of what the PCs will be up against.

You can always do the 'villain' is really the good guy bit.

Common tropes are:

• Time traveler sent back in time to kill Hitler
• Freedom fighter taking back lands.  Your town is built on the holy lands of the Orcs.  Those are OUR houses you live in.   You pushed us back and stole our homes, now we are here to take them back.   We do not demand your death, just our property.  We will not kill you if you leave peacefully.  Stay on our property and die.
• "Are we the baddies?  Why do we dress all in black and have skulls on our helmets?"    Your mission is to remove all kobolds from his western lands.  When they get there you find they have no weapons, refuse to attack and simply sneak back.

Gives the party some moral dilemmas to think about.  See which of the players man up and turn against the mission.

As soon as you described the crystal breaking and turning people into heroes, I pictured other less honorable people twisting into monster shapes at the same time.

Maybe magic warps your outsides to match your insides. Would be a nice way to give yourself a bit more variety in combat, and to ensure players may get use of abilities that target those other creature types.