In order to get rid of spring fever, I used to swim or ride my bicycle before COVID-19.

Before COVID-19, I used to swim or ride my bicycle in order to get rid of spring fever.

Now, drinking tea is only way to prevent tired.

Now, drinking tea is the only way to prevent tiredness. (my only way to relieve tiredness)

My friend suggested me to eat vitamin D supplements, because we weren't under the sun daytime.

My friend suggested that I take vitamin D supplements, because I wasn't in the sun during the day. (we aren't in the sun)

I will try to eat it tomorrow.

I will try to take some tomorrow.

I assume that it is becaused by of spring fever.

Your sentence isn't wrong but using "because of" sounds better

Now, drinking tea is the only way to prevent feeling/being tired.

My friend suggested me to eattaking vitamin D supplements, because we weren't underin the sun daytime.

When talking about medicine, supplements or vitamins we use "to take"
e.g I take my medication every night

He said the vitamin D is good for vitality.

I will try to eataking it tomorrow.