To get through early levels, try clearing from the bottom left. This will not maximize score but is a good way of clearing the level quickly.
Remember that different colors earn different special blocks. Focus on green blocks to earn multipliers for higher scores and yellow blocks to earn lightning blocks to help expand the size of your clears.
Clear out one color at a time, to try to maximize the last color cleared into as large a group as possible.
Marathon:
Use wild blocks in Marathon mode to connect large groups of blocks including multipliers and booster blocks. Set up large clears to earn more wilds.
Conserve your wild blocks for later levels when it gets harder to make groups.