Hi, Eddie. You've come to the best site on the Internet for model railroading! There's a lot of friendly and knowledgeable folks here and this is just the place to get answers to all of your questions.
I model in N scale but will try to answer a few of your questions. HO products should be compatible as far as mixing items from different manufacturers to run on the same track (rolling stock, etc.). Most new equipment comes with Kadee couplers or an equivalent that will work with other brands. If you buy older RR cars, they may have "horn hook" couplers on them which only work with other horn hook couplers. Google model railroad couplers.
There are kits out there, but I don't do HO so............. Newer Bachman equipment has been pretty good for the money.
You need to do a lot of reading and research your purchases well to get good equipment for a fair price. Also, ask questions here............don't be shy............no one here bites!
"N", the Normal scale!