You are abouit to see how expensive this hobby CAN become!
WRITE down your preferred railroads, area modeled and time period. Make a list of engines that would operate in your locale in your time period
If you model CSX or NS in 1990 in central Georgia, do not buy a Pennsy GG-1 or a Sante Fe Big Boy steamer. Old Southern, N&W, etc diesels can be considered as they are predecessers to NS.
Consult your list, if it does not fit your list, do not buy it. You will save thousands of $ you can spend on stuff you find that does fit the list.
There is a thread on here with many favorite train websites.
I check the HO clearance section of M.B. Klein (modeltrainstuff.com) every couple weeks for engines on sale. Look for engines with DCC on board at least, and some have DCC and Sound installed.
Write down the road name, manufacturer, stock number and price. Be sure to check the forum "for sale" section here, too.
Stay away from TYCO, IHC and LIFELIKE until your are experienced enough to judge an engine's quality
Rolling stock can be pretty eclectic for freight , so use all types of cars and road names, IF THEY FIT YOUR ERA. A Nickle Plate caboose and Rio Grand Southern baggage car are not your CSX era freight rolling stock.
HOWEVER, it is your railroad and you run what YOU want on it.
The most important thing is for you to enjoy it. Try to keep from buying everything that looks good to you, because most of it looks good!