Thanks for everyone's replies!
Just to be a bit more clear, I'm not talking about house plants, just vegetables.
So far I've just put a bit of compost in with the potting soil. I didn't want to put any in because I read that it needs to be nice and fluffy for drainage, but the guy at the green house who sold me the potting soil said it would be a good idea to put some compost in as well. I am sure the potting soil has fertilizer in it, most bagged soils come with enough nutrients to feed for a few weeks to a few months.
It's not that I have an "issue" with fertilizer, I just don't know much about feeding a garden so I was wondering if compost would be as effective, as it is definitely the more natural of the 2, so if it would have been just as effective I would prefer to use compost. But if it's not the same I would just use an organic fertilizer. No problem.
The reason I used mushroom compost is because I went back to the green house as I ran out of compost and I asked for a small bag. The girl brought me that and said it was the smallest bag they had and I had a feeling it wasn't good but my kid had already had enough and I had to get out of there quick so I just took it. I didn't mix any of it in to any of my plants though, just sprinkled it on top. So it shouldn't be a problem right? And I plan to use these year after year so I'm sure the bit of compost inside won't be too bad. I hope anyway.
At the end of the day I'm sure I'll get some vegetables whether I do things right or not and I'm just experimenting. My moms always been the type of gardener to just throw the seeds in the ground and water and nothing else, so that's all I knew. Until I started doing my research over the winter I had no idea about 3 sisters planting, setting a trellis up for your peas, how to prevent your tomatoes from splitting etc...so I have just learned so much stuff, I'm kind of in experimentation mode here I'm sure I won't get it all right this year but that's ok. I'll continue learning!
One more question! Is it pretty hard to over fertilize or is this something I really need to be careful with?