Obviously, you're not going to be able to say everything you want to say with the top 100 words in Spanish—although you could do surprisingly well with fewer than 1,000. But if you can learn these words and understand how they're used, you'll be a long way toward being able to communicate freely in Spanish.
Definitions below are for quick reference; all the words can be translated in additional ways.
Top 100 Spanish Words
1. gracias (thanks)
2. ser (be)
3. a (to)
4. ir (to go)
5. estar (to be)
6. bueno (good)
7. de (of, from)
8. su (your, her, his, their)
9. hacer (to do, to make)
10. amigo (friend)
11. por favor (please)
12. no (no)
13. en (on, in)
14. haber ("to have" as an auxiliary verb)
15. tener (to have, to possess)
16. un, uno, una (a, one)
17. ahora (now)
18. y (and)
19. que, qué (that, what)
20. por (for, by)
21. amar (to love)
22. quién (who)
23. para (for, to)
24. venir (to come)
25. porque (because)
26. el, la, los, las (the)
27. antes (before)
28. más (more)
29. bien ("well" as an adverb)
30. aquí, allí (here, there)
31. querer (to want, to love)
32. hola (hello)
33. tú (you)
34. poder (to be able)
35. gustar (to be pleasing)
36. poner (to put)
37. casi (almost)
38. saber (to know)
39. como (like, as)
40. donde (where)
41. dar (to give)
42. pero (but)
43. se (itself, herself, himself, themselves)
44. mucho (much)
45. nuevo (new)
46. cuando (when)
47. chico, chica (boy, girl)
48. entender (to understand)
49. si (if)
50. o (or)
51. feliz (happy)
52. todo (all, every)
53. mismo (same)
54. muy (very)
55. nunca (never)
56. yo, me (I, me)
57. sí (yes)
58. grande, gran (big, great)
59. deber (to owe, should)
60. usted (you)
61. bajo (low, under)
62. otro (other)
63. salir (to leave)
64. hora (hour; see also lesson on telling time)
65. desde (from)
66. ver (to see)
67. malo, mal (bad)
68. pensar (to think)
69. hasta (until)
70. tanto, tan (used in making comparisons)
71. entre (between, among)
72. durante (during)
73. llevar (to wear, to carry)
74. siempre (always)
75. empezar (to begin)
76. él, ella, ellos, ellas (he, she, they)
77. leer (to read)
78. cosa (thing)
79. sacar (to take out, to remove)
80. conocer (to know)
81. primero (first)
82. andar (to walk)
83. sobre (over, about)
84. echar (to throw)
85. sin (without)
86. decir (to say)
87. trabajar (to work)
88. nosotros (we, us)
89. también (also)
90. adiós (goodbye)
91. comer (to eat)
92. triste (sad)
93. país (country)
94. escuchar (to listen, to listen to)
95. hombre (man)
96. mujer (woman)
97. le (indirect-object pronoun)
98. creer (to believe, to think)
99. encontrar (to find)
100. beber (to drink)
And a Few More
Here are some other words that very well could have made the list of the top 100 words in Spanish:
101. hablar (to speak)
102. ese, esa (demonstrative "that"; also see the demonstrative pronouns)
103. baño (bathroom)
104. después (afterwards, later)
105. gente (people)
106. ciudad (city)
106. sentir (to feel)
107. llegar (to arrive)
108. pequeño (small)
109. escribir (to write)
110. año (year)
111. menos (minus, except)
112. lo (various uses)
113. cual (that, which)
114. este, esta (this)
115. dejar (to leave)
116. parte (part)
117. nada (nothing)
118. cada (each)
119. seguir (to continue, to follow)
120. partir (to divide)
121. ya (still, already)
122. parecer (to seem)