Should I use capital letters in titles?

Both are fine!
My advice: use small letters.

Many years ago students were taught to use capital letters in titles. For example:

The Effect of Changing Sleep Patterns on Brain Function

But this is changing, at it is now perfectly OK to use small letters in titles. For example:

An economic assessment of syrup production in Brazil

One reason for this change is that certain things in science are always with big or small letters, like mRNA or E. coli, and this makes using capital letters in titles complicated.

Whichever you choose (capitalised or uncapitalised titles), use only one style and use it throughout your text.

Good:

Materials and methods

Results and discussion

Also good:

Materials and Methods

Results and Discussion

Bad:

Materials and methods

Results and Discussion