First, have a look at what the people at the other tables are eating, and make note of what appeals to you.
Make sure the place has a Menu del Dia, and/or a Menu Peregrino... and make it clear you want the Menu del Dia. (it's what the Spaniards all are eating, for good reasons). The staffer will then rattle off a list of "primeras," first courses. Ask them to go slow, pick a couple that sound good. Bean and lentil dishes are often very good, "menestra" roast veggies always a good bet, but pasta? Not unless it's an Italian place.
The staffer will want you to choose a starter right off the bat, but I like to see what's on offer for Segundas, and match up the two to comprise a balanced meal. Watch out for "lomo," "filete," and "rebozada.." which can sometimes mean "sliced deli loaf," "fried shoe leather," and "deep fried fat" respectively. Fish is a good choice. Presa, solomillo, secreto, carrilladas, codillo, pechuga, and cuarto are usually good things, meat-wise! Once you've heard the Segundas list, tell 'em what you want... occasionally I'll opt for two starters, and that's never a problem. If there's no salad offered and you really want one, you can ask if that's possible.
The person will come back after for your dessert order. Always ask what's "de casera," made in-house. Have em tell you what the fruit options are, as fresh dessert fruit is often superb in Spain.
If nothing sounds great, have a look at the "platos combinados," or just order a big "racion" of something you know you like.