i have comleted mylevel 5 but i can notunderstand how i will start new levels please help me
Your levels will automatically increase as you gain more XPs. You can go through the table present in the link below to check amount of XPs needed to achieve a particular level.
For proceeding with the tree, you need to complete all lessons in the available skills before the next set of skills are made available. In your case, you need to complete the Questions skill before the next set are available.
Levels are just a measure of how much overall XP you've accrued in a language. As long as you keep completing new or strengthening lessons you'll be gaining XP. It looks like you only have 140xp more to get until you reach level 6 in Spanish.