The beggining stages of a (non bounty) tournament should be played much like a cash game, because you have a lot of blinds and your tournament life has little value at this stage. It is a much better strategy to attempt to play anything that wins you chips early on and take thin edges than to sit there and lose your blinds. You can actually steal a little looser than a cash game because of the antes making this more profitable.
As the bubble approaches, you need to be a little less willing to call off big portions of your stack with marginal holdings so you can make it into the money. You also need to pressurise other people with medium stacks who can’t call off easily by being aggressive against them. However. you shouldn’t take this too far because making it into the cash isn’t your main objective. All the money in MTTs is locked up at the top.
After you’ve made it into the money, you can start to play in a more ‘normal’ way until the final table approaches. This is due to the fact that the initial ladders are almost irrelevant. Bear in mind that your stack will be worth less big blinds by this point so (depending on the tourney structure) you will need to be willing to risk your chips and not lose the blinds.
Once you make it to the final table ICM becomes a huge consideration. As a short stack, you need to be loose and try to get a chance at making a stack again, because you are unlikely to outlast other people by sitting there and waiting. As a medium stack, it is essential to try to outlast other players with medium and short stacks. The amount of
equity you’d lose by busting at this stage is far more than the equity you gain by doubling up. As a big stack, you need to be very loose, especially against the medium stacks who have to be tight against your raises.
I hope that makes sense. It’s fairly brief because this is a huge topic but I think that will point you in the right direction. If you want more info I’d recommend researching how ICM works.