What's the difference between ticking booking for roundtrip from US to Nepal through online by myself and by tavel agency that we can find here in US. I have around 3..5 months remaining for my planned flight. Is there anything I need to consider while booking online by myself. Thanks in advance.

It's usually the same. The agencies may have answers to questions you might have regarding visa or something other. Sometimes they might also be able to get you deals. When looking for a ticket, open up incognito mode on chrome browser so that websites are not able to track you with cookies. Compare the prices online yourself and then talk to local agencies. If you can get a cheaper ticket yourself, get it. 

thanks maxpayne..

Some times online tickets are cheaper than the ones quoted by travel agents, and other times, travel agents are cheaper. The travel agents can provide you with complementary hotel deals for certain travel routes, usually for longer layovers and this usually does not happen with online bookings. Also, depending on your departure city, online travel sites may not pull some flights/routes, something travel agents are able to do that gives you more options on the flight route/airlines and pricing. 

Travel websites quoting cheaper rates than travel agencies is a rarity, especially for International travel. From my own experience, skyscanner.net and cheapoair are two of the best. 

You could simultaneously try purchasing your tickets via travel websites (make sure you've cleared your browsing history/cache,though) and travel agencies. I purchased mine via AeroNepal and their price was $40 cheaper than what I was quoted on cheapoair. 

Since you've got all the time in the world prior to your departure, you could try calling/emailing various travel agents. 

Dai, try to get a quote from Nepal as well. I went to Nepal twice - once I got ticket online, another through a travel agent (Nepal Travels ki ke ho). I don't know what kind of tickets they got for me, They said their system showed that I could carry two luggages, but I was charged for the 2nd luggage by the airlines. Nepal Travels promised to refund me the extra charge, but they never did... I followed up for 4 months, and then just got tired of wasting my weekends on phone.

Thanks everybody for suggestions and sharing experiences. Those were useful.
I tried to get the quote from agency in Nepal and did try online also. It was not that great price from agency in Nepal but have not tried to get quote from agency here in US (but will do).
So how early is too early. I have 3.5 months left for my planned travel?

Better yet don't travel save money