Pretty much the standard rule for cooking fish any way is 10 minutes per inch of thickness of fish. Baking is usually longer depending on how high the heat is but grilling and broiling follow pretty close to this rule.

Fish is easy to cook because when it is done it looks done.

Tilapia is one of the cheaper fish you can find anywhere. It has a mild flavor and hold together pretty well so you can do a lot it, grill bake or broil. Simple healthy recipe like some lemon butter sauce and some herbs and broil it and your good to go.

Salmon is amazing and sometimes a good deal. Salmon can hold up to sauces and flavors very well. You can marinade and grill it or bake it. Farm raised usually means they live in a netted in region that is connected to open water (not a inland closed water system) and there diet is controlled. Wild salomn can be pricey, and I would recommend only buying fresh salmon.
Salmon can usually be purchased as steaks or fillets both are good for grilling. If you do plan to grill fish frequently invest in a grilling basket it can make grilling easier.

IMO the best thing to do is talk to the person you are buying from and buy what is fresh and they just got in this is what will taste the best. No fish is really too hard to cook and you can find recipes for anything online. At least were I work we have sales on different fish every week and that tends to be our best stuff since we sell so much and get more every day. SO best thing is just to talk to the guy selling the fish.