Something wrong with acceleration?

I have a '92 Ford F150 XLT supercab that ran nearly perfectly when I bought it (for very
cheap.) My dad figured the fuel filter needed replacing (Why? no idea) so we did that and
the front fuel tank leaked so we replaced it with a new one. We also changed the oil,
greased the front and a couple of other things; I don't recall specifics. When I got it, it ran
well. It idled fine, it accelerated perfectly, the brakes were fine. It did take a while to start
but I expected that in the middle of winter in Minnesota (especially this year!) After my dad
and I worked on it, the brakes lights are on (though there doesn't seem to be a noticeable
problem), it doesn't accelerate well, and it seems to take longer to start than it should. It
sounds fine idling. Also, I have not noticed any leakage of any kind from anywhere. (I'm
new to any kind of vehicle maintenance but please don't hold back! I'm a quick learner.)