M.A

The qualification for the NET exam is minimum 55% marks in Masters degree from any recognized university or institute of India.
As you have completed the MA in History, if you have got more than 55% marks in your Masters degree than you can surely apply for the NET exam.
There is 5% percentage relaxation for the reserved category candidates.
I found a blog where, you can check the complete eligibility criteria in detail from the link given below:
https://scoop.eduncle.com/ugc-net-exam-eligibility check in details here.
The NET exam is conducted for the selection of the candidates for assistant professor posts or junior research fellowship in top most institutes of India.
There is no age limit for the assistant professor posts while candidates applying for Junior Research Fellowship should be below 28 years old.
Also you can check official website for more detail about the latest information regarding the NET exam.

The level of difficulty in MA Economics is actually depends upon you. If you didn't have studied maths at 12th level or if you don't shows interest in mathematics then it would be very much difficulty for you (Difficulty level can be 09/10). Because maths is the essence part of economics. Without the knowledge of maths, you won't be able to find the 'Time Value of Money', Bond and Stock valuation, Price determination under different forms of market, Managerial Economics. All these requires higher level knowledge of calculus, algebra and probability. The concepts of differentiation and integration helps you to formulate the policies by knowing the maxima and minima. If you are from non-maths background, then I will recommend you take tuitions of advance maths for your better performance during college. Otherwise it will be very difficult for you.
I hope this helps.
Best of luck.

Follow the tips mentioned below:
1. Read two newspapers every day, EVERY DAY- I would suggest the Hindu and Indian Express. Online or whatever. Read Editorials and Op-Ed properly.
2. Read Frontline every week cover to cover
3. Start reading fundamental concepts and theories (see point 6)
4: Brush up on India's political history (India after Gandhi by Ram Guha is a good start)
5: Read Economic and Political Weekly sometimes. Try to see how essays, arguments, data is used. Some of it you may not understand, but just read to see patterns, structures, language. Also read Seminar magazine Seminar Magazine, a monthly in-depth magazine, published by Seminar Publications, New Delhi especially January issue for past 4-5 years.
6: Get basic textbooks like those by Andrew Heywood. Ensure you do concepts, theory, and philosophy.
7: Acquire a copy of Oxford Companion to Politics in India (Students Edition) Read as much as you can. You may not understand all of it, but get going :)
8. Take time after a lot of reading to sit and write some argumentative essays.
10. For writing answers in the Entrance you need the following:
Step 1: You need ideas - that is where theories and concepts come in
Step 2: You need knowledge about events and the political history of India and the world.
Step 3: You need to combine 1 & 2 into coherent structured essays. Learn how to support your arguments and opinions with facts, theories, and concepts.

Admission to MA program in English ideally begins around the month of February and March when Universities begin to release their application forms. Generally universities conduct entrance examination for admission to the MA program in English but there are certain universities which take admission on the basis of merit. DU grants direct admission to students who have completed their Bachelor of Arts degree in English Literature from any of the colleges affiliated with DU with an aggregate of 60% or more marks for General category candidates and 54% marks for OBC/SC/ST/PwD candidates. 50% of the total seats are reserved for direct admission.
Apart from that, all the major MA English offering universities conduct entrance exam for admission of students to the program.
GGSIPU
JNU
DU-MA entrance exam
BHUEE-PG
and many more.
The preparation required is quite similar only the pattern of the exams are a little different.

Yes, you are eligible for the NET examination as per the qualification.
The eligibility criteria for the National Eligibility Test is post graduation degree with minimum 55% marks from any recognized university or institutes of India.
As you are post graduated in MA, you can appear for the NET exam. If you want to go for the fellowship award through the NET exam then you must fulfill the age limit criteria. The candidates must be below 28 years old. There is 5 years age relaxation for the reserved category candidates.
There is no age limit criteria for the the assistant professor posts.
The NET exam is proposed to conduct twice in the year, although it is not confirmed yet. So be updated with the latest information, visit official website for more information. All the best for your future.