Ties and slurs both draw a curved arc between notes, but they mean different things. Here's how to tell them apart and use each one.
At a glance they look the same, but the musical meaning is completely different.
Connects two notes of the same pitch into one long sound. The second note isn't played again — the first note just keeps ringing.
Marks a group of notes to be played smoothly (legato). Unlike a tie, a slur can connect notes of any pitch.
See the difference in the image below.

Select a note and press T. That's all there is to it.
T works as a toggle — press it again to remove the tie.
After pressing T, it should look like this:

A few things to keep in mind:
Select the note where the tie starts, then press T again. You can also right-click the note and choose "Remove Tie".
Select the note where the tie starts, then press T again.
Right-click the note and choose "Remove Tie" from the context menu.
Select all the notes you want to slur, then right-click to add it.
Tip: You can also right-click a single note to mark a slur start, then right-click the final note to mark the slur end.
Right-click the selection and choose Add Slur, as shown below.

Select any note inside the slur, right-click, and choose "Remove Slur".
Here's a quick exercise to practice both.