I'm running.

"be-verb+ ...ing" is so called a progressive form.

"...shiteiru" in Japanese is almost equals with a progressive form in English.

When "...shiteiru" is attached to a verb, the verb should be conjugated.

hashiru : run
hashitteiru : be running

Watashi wa hashitteiru. : I'm running.
Watashi wa hashitteimasu : I'm running.(polite)

taberu : eat
tabeteiru : be eating
tabeteimasu : be eating(polite)
nomu : drink
nondeiru : be drinking
nondeimasu : be drinking(polite)
yomu : read
yondeiru : be reading
yondeimasu : be reading (polite)
miru : see, watch, look
miteiru : be watching
miteimasu : be watching (polite)

There is a little bit of difference of nuance between " ...shiteiru" and " be ...ing".
But that is not a big problem.