Problems with changing car battery?

Prior to battery change, the car had to be boosted for it to start but otherwise started no problem. Took out old battery which was top mount. Replaced with new battery which is side mount but bought those adapters. Now there is power in that the lights and radio come on but the car won't start. Tried boosting it but still nothing. It doesn't even try to turn over. It is a 1998 Plymouth Neon. Does anyone have any idea what is wrong? Thanks - could use any suggestions.