Greetings aspirant,

First impression is the last impression. In every recruitment process, the written part plays a very important role as the marks from the test gives your primary impression to interviewer.

Speaking of TCS, it actually depends for the post you`re applying for. Their aptitude section is actually quite difficult, but no need to worry, just a month or two of regular and serious practice shall make you skilled enough to go through it smoothly. There are english, programming section as well. These are pretty basic sections. It is a computer based test so make sure you type fast enough. Keep the basic coding in practice as it might come handy. There is no specific cutoff marks, but there is percentile as per their job vacancies and other requirements. 

Do your best, stay focused.

I remember on the day of my TCS interview, I was tensed. The recruiter told us not to get tensed and under NO circumstances we must stay empty stomach and dehydrated.

Hope it helps.

Good luck for your future endeavour!