There are several ways to make a (part of a) text bold.
If you use the Source Editor you can select the part of what you want to make bold and click on the "B" on the top of the editor. If you use the Visual Editor you can do this also with the "B" on the top.
Another way is using this code:
'''Text here'''. It'll result in this: Text here.
If you want to use tags you can do it on this way:
<span style="font-weight: bold;">Text here</span>. This will result in this: Text here.