98 chevy has to be jumped to start everytime

I have a 98 Silverado that i got off a friend. she had wrecked it but still drove it a few months before her ole man made her park it and it set a couple years. went to go get it put a battery on it and some good gas, I still had to jump it but it started and drove 60 miles no problem. got home shut it off and went to restart it and click shows battery is dead light barely on. next day jumped it again ran good just couldn't turn it off. then next day I jumped it and went to store and it died and wouldn't start. got it home and put jumper cables on it and it would click sometimes it would act like it was turning motor real slow then quiet. still have to jump every time but its getting harder to get it to start and once started it'll sit and idle all day but if u drive it u might drive around for hours or might go 10ft before it dies. the battery supposed to be good just bought it a couple weeks ago I put another alternator on it and took starter off and tested it its good. the last time I drove it was at night and I all of a sudden all the lights {headlights and dash lights} start dimming down the brighten up over and over till it died again.