I only conduct my study with one intervention group without a control group. To compare this group’s pretest and posttest scores, what t-test should I use?

Yes

I see.

The paired-sample t-test. The Paired Samples T-Test compares the means of two measurements taken from the same individual, object, or related units. In your case, the "paired" measurements are taken at two different times (e.g., pre-test and post-test scores with an intervention administered).