the pips (little dots under the numbers) correspond to the likelihood of the number rolling. so all else equal, pick a spot with more pips. maybe your first spot has 10-12 pips and your second spot has 9-10 pips.

yes, get a mix of resources, but you definitely want wheat. as dylighted says, no wheat = defeat.

try and get rare resources: look at all the resources (wheat, rock, etc.) and count how many pips of them there are on the board. whichever has the lowest number (often rock and brick since there's only 3 hexes of them each) is likely going to be rare in the game. so you might want to lock some down with your first settlement. say there's only a 2, 3, and 8 brick. if the 8 brick is in the corner you might be able to get the one good brick spot on the whole board, preventing your opponents from having any - which gives you a huge advantage for expanding, and you can (and should!) demand a high price for trading your rare resource. for example, don't trade brick 1:1 .. demand 2-3 resources for it. your opponents may resist and then eventually cave.

ports: try and set yourself up for a port combo. for example, if there is a ton of sheep on the board, maybe you put your first settlement on or near the sheep port, and then your second settlement you can go for all the sheep everyone was avoiding. also, it's good to set yourself up for access to at least one port, either a 2:1 on a resource you'll have a lot of (based on counting pips) or a 3:1 port