Should I start by getting a better amp, guitar or expanding my pedals? Unfortunately I'm a student and don't have that much money to spend(although I'm quite willing to skip some weekends partying to save money).
oh and sorry if my english isn't perfect
You're English is fine.
Your Big Muff is a good place to start. You may want to experiment with other fuzz pedals, but definitely keep the Big Muff. It's a great pedal.
I'm sure a lot of people will tell you to get an Orange or Matamp amp head. While these are excellent amps, they are very expensive. One alternative I've found is the Tech 21 Oxford pedal. It duplicates the tone of an Orange amp, at a fraction of the price.
Another important thing to consider is the speakers you're using. Personally, I think you need at least a 4x12 cabinet. Two of them is even better. There's something special that happens when the sounds from multiple speakers mix in the air - your tone has more depth and complexity. Celestion speakers are generally considered the best, but I also like Peaveys a lot, especially Peavey Sheffield speakers.
Also, a lot of the tone that doom players get is a result of high volume. The louder you play, the more sustain and feedback you'll get.