But the sad truth is, you won't get a deal with a long novel as a first timer. Paper costs money and it's twice as expensive to print a 150,000 word novel as a 75,000 word one. (Most of those mega blockbusters that could double as doorstops were printed in the 1980s when paper was much cheaper than now.)
So you're going to have to cut. This is going to hurt, but there it is. It will have to be done. And to drop 1000s of words is going to require more than deleting a word here and there.
1. Description. Long descriptions of places, people and things are obvious targets for pruning. Start by asking if you really need them in the first place, and if the answer is no, cut them out altogether. If the answer is yes, then cut back - people don't read long descriptions any more.
2. Scenes. Do you really need every scene that's in the book? Write down the purposes of each scene eg introduce Character X, describe the house, give some information about X's backstory. A scene that is short on purposes should be short on the page. If it isn't, then cut - or combine with another scene.
3. Use summary. Sometimes we write out scenes when we could actually use summary. In Nice Girls Do I wanted to describe the C18th landscape garden where the story is set, and had a scene where Will the gardener shows Anna, a garden historian, round. Some of the garden is described through straight descriptive passages, the rest through dialogue between Will and Anna. It was a very long scene - 16 pages in the first draft. I realised that Anna, being a garden historian, would know most of the information anyway, so given she was the view point character I could put much of the information in her head as summary. It's about 6 pages in the published version.
4. Characters. Do you need them all, and if they're important to the plot, do they all need to be described? You may have worked out detailed character histories for each and every one, but it's not essential for the reader to know all this stuff. Think iceberg - 10% above the water, the rest submerged.
5. Back story/flashback. How much do you really need, and how much is you writing yourself into the characters? Most of the time we don't need to know about what happened in the past and it just holds up the story.
6. The opening chapters. Have you started in the right place? Where does the story actually start? It's not unusual for the story to really get going at Chapter 3.
7. Too many sub-plots? Perhaps one (or two) could go. Always ask yourself what their purpose is in relation to the main story line. Remember that readers can't handle more than five or six main characters in a story. Usually there are two or three who are centre stage most of the time, with supporting characters coming in and out of the main story focus.
Most novels currently published are between 80,000 - 100,000 words long. If yours is way over that, you're just going to have to be brutal.
NB If having read this your immediate response is Trilogy! I would respectfully suggest you think again, for reasons I'll deal with another time.