Team Lead is a company role, plus something you can be on a team. You assign and review your team's work and keep it moving. Everything a Team Member can do, plus distributing work.
There are four system roles. Team Lead is a company role (rank 40 by default) whose system role is Member.
The default Team Lead role carries: create tasks, assign work, edit work, view internal items, and view activity logs. An admin can change this list. It does not include managing projects, so you see only the projects your teams are on or that you were added to, and you can't decide task reviews unless you are also a Manager on the project.
Separately, you can be a Manager of a team. On the Teams page, that lets you add people to your team, remove them, and make or unset team leads. It is set by an admin or another team manager.
Access to projects follows teams. When someone adds you to a team, you get a notification and a banner at the top of the page with Accept and Decline. Nothing is shared with you until you accept. You can be on several teams.
You can open a project if you own it, were added to it individually, or are an accepted member of a team assigned to it. A pending or declined invitation grants nothing. A project you can't see shows a "not found" page on purpose.
Your role needs Assign work, and you need to be able to see the project. Ask an admin.
Only a team Manager or an admin can. Ask one to make you a lead.
Click your avatar (top-right) → Profile & preferences → Change password. Starter passwords arrive by email, so change yours on first login.