Can I wear hijab only for prayer at first?

Covering in prayer is required

For a woman who has reached puberty, covering the whole body except the face and hands (and the feet in the Hanafi school) is a condition of a valid prayer. The Prophet ﷺ said that Allah doesn't accept the prayer of a woman who has reached puberty without a head covering (Sunan Abi Dawud 641). So a prayer scarf or prayer outfit isn't optional, even when you pray alone at home. See What do I have to cover when praying at home alone?

Hijab outside prayer is also an obligation

Scholars from all four schools agree that hijab in front of non-mahram men is required, not just a prayer outfit. Being honest about that matters. At the same time, a new Muslim learning many things at once doesn't have to do everything on day one. Most converts start with the shahada and prayer, then add other practices as they grow. See Do I have to wear hijab right away after becoming Muslim?

Why starting with prayer works

• It gets you used to the feel of a scarf and how to wrap it.
• It ties hijab to a moment of closeness with Allah, not pressure from people.
• The Prophet ﷺ said the most beloved deeds to Allah are the most consistent ones, even if small (Sahih al-Bukhari 6464).

Prayer itself also builds gradually for many new Muslims. See Do I have to pray five times a day right after becoming Muslim?

Moving forward from here

Treat prayer-only as a starting point, not a final stop. Try wearing it for the walk to the mosque, then for a short errand, then for a whole weekend. Set yourself a gentle goal, make dua for strength, and talk to a trusted sister about what feels hard. Every time you cover, you're obeying Allah; any gap is something to keep working on, not a reason to give up.