It's a good thing there are trade shows around to help us make life's difficult decisions:

* If you're thinking of marriage. You can learn everything about throwing a wedding at the Fourth Annual Washington Wedding Show this Saturday and Sunday from noon to 6 at the Sheraton Hotel and Exhibition Center in New Carrollton. Ninety companies will be on hand to exhibit wedding items and services, including catering, photography and even renting a Rolls-Royce. There'll be two bridal fashion shows each day, at 1 and 4. Tickets are $5 at the door; children under 12 free. The SheratonHotel is at 8500 Annapolis Road (Beltway Exit 20-B), New Carrollton, Maryland. If you're taking Metrorail, exit at the New Carrollton station from the Orange Line (three-minute bus transfer to hotel). For more information, call 441-1303.

* If you're thinking of a horseless carriage. Dealers have rolled 500 brand new autos into the Washington Convention Center for your inspection at the National Capitol Area International Auto Show. Cars from 29 countries, from the no-frills Yugo to the royal-thrills Rolls, will be on display this Friday from 11 to 10:30; Saturday, 10 to 10:30 p.m.; Sunday, 11 to 10:30; and Monday 10 to 8. Tickets are $5 for adults; $2.50 for children six to 12. Call 371-3112 or 657-1800.